Recovery of Macro and Micro-Nutrients by Hydrothermal

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Recovery of Macro and Micro-Nutrients by Hydrothermal Carbonization of Septage Kyle McGaughy, and M. Toufiq Reza J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05667 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Journal of Agricultural and Food Chemistry

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Recovery of Macro and Micro-Nutrients by Hydrothermal Carbonization of

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Septage

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Kyle McGaughy1, M. Toufiq Reza*1, 2 1

Institute for Sustainable Energy and the Environment, 350 West State Street, Athens, OH 45701, USA

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Department of Mechanical Engineering, 1 Ohio University, Athens, OH 45701, USA

* Corresponding Author. E-mail: [email protected] Tel: +1-740-593-1506

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Abstract

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In this study, septic tank waste, septage, was hydrothermally carbonized (HTC) in order to

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recover macro-and micro nutrients, while tracking the fate of residual heavy metals. Three different HTC

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temperatures i.e., 180, 220, and 260 °C at autogenous pressures and two reaction times i.e., 30 and 120

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minutes were applied on both solid and liquid septages. Hydrochar and HTC process liquids were

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characterized using ICP, CHNS, and UV/Vis spectroscopy. Treatment at 260°C for 120 minute

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maximized ammonia recovery, producing a liquid with 1400 mg/L of ammonia. Overall, about 70% of

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available nitrogen ended up in the liquid phase as nitrate or ammonia. Solid hydrochars show potential for

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fertilizer use, with high phosphorous content of 100-130 kg/tonne. It was found that heavy metals mainly

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remained in the solid phase, although the concentrations of heavy metals are mostly lower than US EPA

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regulation for biosolids with the exception of selenium.

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Keywords: Hydrothermal carbonization, septage, NPK fertilizer, phosphorus, heavy metals, ammonia,

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struvite

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1. Introduction

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Approximately 10-15% of populations in the world use some form of household sewer

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treatment system (HSTS) or septic tank system 1. While septic tanks are not as common in the

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US as they once were, till now about 1 in 5 houses have some sort of on-site sewage treatment

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system 2. Septic systems are more economical for areas where centralized waste treatment is too

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costly due to sparse population or difficult terrain 3. Unlike wastewater treatment facilities,

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where chemical additives (coagulants and flocculants) are applied to treat sewage, septic tanks

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are stand-alone systems that use partial anaerobic-aerobic digestion to breakdown sewage. This

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digestion is a result of residual enzymes and bacteria in the septic system. Organics such as

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waste oils and fats separate from the aqueous phase and form a floc layer on top of the tank

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called the scum. Meanwhile solids settle in the tank and are digested in an aerobic-anaerobic

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environment. Over the years, the solid and scum layers accumulate, septic tank reduces its

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efficiency, and cleaning becomes inevitable to avoid septic tank failure.

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Typical septic tanks in the U.S. range from 3000-6000 liters in size depending on house

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size, number of occupants, and location. These septic tanks require periodic cleaning; typically

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between 3 and 10 years, as suggested by the local county health department or EPA. During

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cleaning, the septic tank is stirred and the tank’s solid and liquid contents are sucked into a

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septage hauling truck. Septage hauling companies usually charge $200-500 to homeowners for

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septic tank cleaning, depending on the size, location, and complexity of the tank 3. This septage

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is then either treated in a nearby wastewater treatment facility or landfilled, depending on

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counties. Therefore, the current practice of septage treatment does not utilize residual energy and

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nutrients. The solids found in septage contain much higher nutrient content than sewage sludge,

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for example 3.7% phosphorus by mass 4. Additionally, energy can be recovered by waste



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incineration, while the subsequent treatment of its ash can remove heavy metals 5. Therefore, to

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utilize all possible resources, the treatment process of septage should be reevaluated to better

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take advantage of its opportunities.

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Hydrothermal carbonization (HTC) has already been performed on various wet waste

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feedstocks, including cow, chicken, and swine wastes, municipal solid wastes, and sewage

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sludge. Research has shown that HTC temperature and reaction time have a significant impact on

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the composition of product phases and can produce usable fuels, soil additives, and platform

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chemicals 6–9. HTC typically carried out at around 180-300 °C and at autogenous pressures, as

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subcritical water in the environment becomes reactive due to high ionic product, low density, and

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low dielectric constant. The appeals of HTC to the wet feedstock, as it does not require

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expensive drying prior to the treatment and it does not require additional chemical additives.

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Hydrothermal treatment has been shown to break down compounds typically found in antibiotics

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and hormones, making it even more appealing for treatment of septage 10. Additionally, HTC

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has recently been shown to produce a nitrogen rich liquid phase in similar sewage sludges 11.

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Due to the negative effects of poorly maintained septic tanks and the cost of properly

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maintaining septic tanks, new research is needed into alternative waste disposal process for areas

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were centralized sewage systems are not practical. One option is to apply HTC on septage in

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order to recover nutrients or generate usable products. To the authors’ knowledge, no research to

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date has been done into HTC of septage. Though, this could result in products such as fertilizers

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or soil additives that may partially offset the cost of septic tank maintenance. HTC usually results

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in a fuel, however, this is not practical with septage due to its high ash content. Therefore, this

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research is focused on HTC of liquid and solid septage and characterizing the liquid and solid

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products for micro-and macro nutrients. Knowing the effects of treatment temperature and time 4 ACS Paragon Plus Environment

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and the composition of the product phases allows for the exploration of additional treatment

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options. It is also important to know the drawbacks of treatment, such as the fate of heavy metal

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species and the environmental impacts of treatment.

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2. Materials and Methods

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2.1 Collection of Septage Samples

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A 20 L aliquot of septage was obtained during the periodic cleaning period for a 1500 gal

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septic system in Athens, Ohio. This sample was drawn from the pumping tank after it had been

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fully mixed. To maintain reproducibility of HTC experiments, liquid and solid septage were

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separated first and added in a certain ratio prior to the HTC. For separation, the 20 L sample was

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allowed to settle for a period of approximately 8 h. The liquid was then removed from the top of

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the container and used as the liquid feed for HTC. The remaining solids were taken from the

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bottom of the container. Solids were dried at 105°C and used as the solid reactor feed. All

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samples before and after separations were stored at -5 to 0°C in a freezer. Samples were thawed

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to room temperature prior to HTC.

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2.2 Hydrothermal Carbonization of Septage

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A 100 mL Hastelloy Parr reactor was used for all HTC experiments. A J-type

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thermocouple was used with a Parr 4848 controller to regulate temperature. An electric furnace

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was used to heat the reactor system. A mixing speed of 180 rpm was used for all experiments.

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Typical heating times were 20-28 minutes, depending on HTC temperature. Typical temperature

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overshoot was about ±2°C for all HTC runs. Reaction time was counting once the controller

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displayed the set temperature. Once the desired reaction time was completed, the furnace was

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removed from the reactor and the reactor was placed in an ice-water bath for rapid cooling. The

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reactor was cooled to 60°C from 260°C within 10 minutes of cooling. 5 ACS Paragon Plus Environment


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Experiments were completed with a constant liquid to solid mass ratio of 10:1.

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Approximately 3g of dried untreated septage was used with 15g of DI water and 15g of septic

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liquid. Experiments were completed at 180, 220, and 260°C for 30 and 120 minutes. Previous

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research into hydrothermal treatment of waste products has shown that reaction times of 30

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minutes are sufficient to achieve a completed reaction, thus reaction times of 30 minutes were

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used for the base set of conditions 9. A longer reaction time (120 minutes) was chosen to ensure

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the completion of HTC reaction 9. These temperatures were selected, as water has its maximum

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dielectric constant in this temperature range. Naming for samples is done by Temperature-Time.

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For example, 220-30 is HTC treatment at 220°C for 30 minutes. All experiments were replicated

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to ensure reproducibility and averages and standard deviations are reported.

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After the reactor had cooled down the mass of all liquid and solid products were

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recorded. There were no significant gaseous products that resulted from the reaction. Ash-free

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eight micrometer-sized Whatman filters were used with a vacuum pump to filter hydrochars

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from HTC process liquids. A specific known amount of DI water was used to wash the

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hydrochars during this filtration to ensure the removal of process liquids from the solid surface.

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Approximately, 100 mL of DI water was used for the washing process, as this was adequate to

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produce a visibly clear filtrate. The filters were then placed in an oven to dry overnight at 105°C.

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The liquid filtrate was weighed and stored in a freezer. The mass of the dried solids was

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subtracted from the total product mass to determine the mass of liquid product. The dilution of

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the liquid product that resulted from the filtration and washing of the solids is considered in all

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measurements and results.


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2.4 Characterization of HTC process liquid

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A Hach-6000 UV/Vis spectrometer (Loveland, CO) was used to determine the

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concentration of nitrogen species and phosphorus in the liquid phase of both the liquid septage

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and HTC liquid products. Hach UV/Vis vials were used for these measurements. These included

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TNT-835 for Nitrate, TNT-830 for Ammonia, TNT-844 for phosphorus, and TNT-880 for TKN

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Nitrogen (including total nitrogen). Cations in the liquid phase were analyzed using a Thermo

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iCAP 6000 ICP (Waltham, MA). Liquid samples were filtered using a 0.45 micron filters before

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analysis. A ThermoScientific Accumet pH sensor (Waltham, MA) was used to determine the pH

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of each sample. The sensor was calibrated at pH’s of 4, 7, and 10 using ThermoScientific pH

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buffer solutions.

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2.5 Characterization of hydrochars

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A Flash-2000 CHNS analyzer (Waltham, MA) was used to determine the elemental

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carbon, nitrogen and hydrogen composition of the solid phase. Sulfur content was found below

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detection limit of the CHNS analyzer (0.5%), therefore, sulfur content from CHNS were not

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recorded. Instead, sulfur contents reported from ICP were reported. 2,5-Bis(5-tert-butyl-2-

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benzoxazolyl)thiophene (BBOT) was used for all calibration curves. The column was operated at

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960°C with helium carrier gas. The column consisted of copper oxide pellets and electrolytic

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copper wire.

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A TGA Q500 (TA instruments, New Castle, DE) Thermogravimetric Analyzer (TGA)

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was used to determine ash content of the solid septage. For this determination a sample of dried

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solid was heated to 105°C for 5 minutes to ensure that all moisture had been removed. Next the

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sample was heated to 900°C with a heat ramp rate of 30°C/min with inert nitrogen gas. At 900°C



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oxygen was introduced for 5 minutes, and the final mass of the sample was recorded once

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combustion had completed.

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Solid products were analyzed using microwave acid digestion in a Mars-6 CEM

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microwave (Matthews, NC). Nitric acid was used for digestion, meaning that only soluble silicon

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would be present in the liquid phase. These samples were also filtered using a 0.45 micron filter

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before analysis. A scanning electron microscope (JOEL JSM-6390, Peabody, MA) was used for

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imaging the surface of hydrochars. Energy-dispersive spectroscopy (Genesis unit, EDS) was

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used to determine the distribution of select species on the surface of the hydrochars.

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3. Results and Discussion

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3.1 Composition of Untreated Septage

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Solid composition data including ash, CHNS, and ICP results are listed in Table 1.

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Untreated solid septage has lower carbon, hydrogen and nitrogen content compared to sewage

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sludge, swine, cow, and poultry manures probably due to the higher ash percentage (46%) 12.

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The ash content of raw septage is much higher than ash content of primary sewage or fecal

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matter, which are usually below 20-25% 13. Septage contained only 30.9% carbon and 4.7%

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hydrogen by mass compared to 36.6% and 5.8% in primary sewage sludge, respectively 14. The

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low carbon and high ash content are preliminary indicators that the untreated solid septage would

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be a poor fuel. Micro and macro nutrients in raw septage are also different than other

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conventional feedstocks. For instance, while nitrogen is similar for septage and swine manure at

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2.9 and 3.0% by mass, phosphorous content is more than doubled than phosphorus content of

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swine manure (3.7% compared to 1.6%) 12. These differences between septage and livestock

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manures are most likely due to differences in diet and biological processing time. Micronutrients,

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especially, calcium and sodium are the most abundant alkaline and alkali metals found in septage 8 ACS Paragon Plus Environment

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at 3.22% and 1.28%, respectively. In terms of heavy metals, untreated septage contains 0.01 wt%

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of lead and selenium, and 0.02 wt% of strontium. These concentrations are within the average

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range of most agricultural soils in the U.S. 15,16.

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The liquid phase composition is presented in Table 2. Chemical oxygen demand (COD)

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of liquid septage is relatively lower (150 mg O2/L) than COD of cow manure and fecal sludge 14.

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Among the macronutrients, a total of 18 mg/L of nitrogen was detected in liquid septage, while

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phosphorus was detected of 1.2 mg/L and potassium was below detection limit of ICP. In terms

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of nitrogen distribution, an 18.1 mg/L of that being in ammonia (14 mg/L N equivalent), 10.1

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mg/L being in nitrate (2.3mg/L N equivalent), with the rest as organic nitrogen, most likely in

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protein-forms. The lower chloride content is consistent with the low sodium and potassium

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content. Small amounts of calcium and sodium are found at 9.43 and 1.5 mg/L, respectively.

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Heavy metals along with the other micronutrients shown in Table 2 were below the detection

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limit of 0.050 mg/L.

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3.2 Solid Composition of Hydrochars

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Mass yield can be defined as the percentage of mass recovered after HTC. Mass yields

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for HTC of septage are presented in Table 1. It is interesting to notice that mass yields were only

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varied significantly with HTC temperature, and not with the HTC time. For instance, solid mass

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yield for 180-30 was 68%, while 180-120 was 67%. At 260-30 mass yield reduced to 60% and

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remained unchanged for 260-120. Elemental carbon and hydrogen followed the similar trend as

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mass yield, where carbon reduced from 30.9% to as low as 25.2% (260-120) and hydrogen

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reduced from 4.7% to 3.5% (260-30). During HTC of lignocellulosic biomass, carbon content

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increases due to the polymerization and condensation of macromolecules found in biomass 17.

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This process results in the dehydration of the solids phase 17. Table 1 shows the solid 9 ACS Paragon Plus Environment


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composition for the hydrochars, and treatment under the stated conditions does not result in

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increased carbon content. Carbon mass % decreased from 30.9% in the untreated septage to

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25.2% in the 260-120 hydrochar. A possible reason for this is that the feed and products ranged

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from pH’s of 6.5 to 9, where HTC is more effective at low pH’s. These conditions may have

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prevented the polymerization and condensation phases and resulted in the lower solid carbon

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content. Elemental nitrogen shows a very different pattern than carbon and hydrogen. In all cases,

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it reduced significantly and the reduction continues for longer residence time, except for 260-30

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and 260-120. In this case, nitrogen actually increased with time. A similar phenomenon was

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observed in previous research when cow manure was hydrothermally carbonized at 260°C 9. The

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authors have suggested that a more porous structure made by higher HTC temperature may have

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entrapped nitrogen at longer residence time. While pH is similar with 260-30 and 260-120, the

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amount of ammonium in the liquid phase is drastically higher with 260-120 and this may

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promote the precipitation or sorption of ammonium onto the hydrochar. Unlike mass yield,

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elemental carbon, hydrogen, and nitrogen; ash content in hydrochar was increasing with HTC

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severity. HTC temperature, however, was more dominant on ash increase than reaction time. For

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instance, around 20% increase of ash was observed from 180-30 to 260-30 but only 6% more ash

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was increased when the reaction time increased from 30 min to 120 min at 260°C. It can be noted

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that the significant increase of ash does not correspond to the mass yield. For instance, the

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theoretical ash would be 77% for 260-120, if all the ash content was inert during HTC.

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Therefore, it is expected that a large portion of ash leached to the HTC process liquid. The

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similar phenomenon was observed and reported for sewage sludge, manures, and whey wastes

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9,18,19

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An increase of ash percentage in hydrochar indicates the densification of nutrients in

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hydrochar. Among the macronutrients, behavior of nitrogen has already been discussed. In

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contrary to nitrogen, phosphorus content was increased in the hydrochar with HTC reaction

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severity. In fact, at 260-30 and 260-120, more than 92% of the phosphorus was recovered in

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hydrochar. At lower reaction temperature, less phosphorus (80% for 180-30) was recovered in

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hydrochar, while more phosphorus was recovered at high temperatures (90% at 260-120). A

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possible explanation is that the porosity of the hydrochar increased at higher treatment

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temperatures, which has been shown to occur in other hydrochars treated at similar temperatures

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its precipitation into the solid phase. Potassium concentration decreases until 220-120 but

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increases afterwards both with temperature and time. A possible reason could be potassium is

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being adsorbed on the hydrochar surface due to the increase of the hydrochar’s oxygen

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functional groups. It has been previously reported that concentration of acidic functional groups

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increases significantly around 260 °C, which are capable of adsorbing cations like potassium 20.

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However, a longer residence time functional groups might be weakened as a result potassium

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concentration was decreasing with longer reaction time for every temperature.

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. Moreover, phosphorous can be evolved in multiple forms and longer reactions times promote

Behavior of micronutrients vary for individual elements. For instance, calcium, iron,

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and magnesium show similar characteristic as phosphorus and potassium, where their

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concentrations increased with HTC temperature but decreased with longer reaction time. A

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possible explanation for these differences is that calcium, iron, and magnesium are only present

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as salts and are being dissolved; therefore longer reaction times mean a lower concentration in

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the solid phase. Sodium and sulfur follow the similar trend as potassium, where the concentration

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decreases with temperature and at 260 °C the concentration is higher than 180 and 220 °C. 11 ACS Paragon Plus Environment


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3.3 Liquid Product Composition Compositions of HTC process liquids from septage are shown in Table 2. The untreated

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liquid has a relatively low COD of 150 mg O2/l. Treatment results in a far higher COD of

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26,000-30,000 mg O2/l. Longer treatment times and temperatures results in a higher COD, which

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agrees with the carbon and hydrogen contents shown in Table 1. HTC results in both mass loss

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of the solid and a lower carbon and hydrogen content. COD of the liquid was the highest for 260-

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30 sample, which lost 40% of its mass and had carbon and hydrogen content reduced from

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30.9% and 4.7% to 25.4% and 3.6%, respectively. Despite there being no significant relationship

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between treatment time and overall mass of the solid phase, increasing treatment time did reduce

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COD. In the 260-30 and 260-120 samples these values are 30,500 and 27,900 mg O2/L.

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In can be noted that compared to HTC of sewage sludge, septage-derived HTC process

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liquids have lower amounts of nitrogen, phosphorus, and potassium. The liquid product resulting

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from treatment of sewage sludge at similar conditions has over 10 times more nitrogen and 5

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times more phosphorus and potassium than the hydrothermally treated septage 8. A possible

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reason for this difference is that septage has already been partially digested in the septic tank,

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resulting in generally lower nutrient content to begin with. When it comes to the trend, total

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nitrogen (sum of nitrate and ammonium-nitrogen) is similar regardless of HTC conditions. In

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every case, nitrogen content is significantly increased compared to raw liquid septage (2030

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mg/L compared to 28 mg/L). At lower temperature, more nitrate was observed, while at higher

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HTC temperature more ammonium-nitrogen was observed. Longer residence time at any given

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HTC temperature, more ammonium-nitrogen was obtained than nitrate. The details on nitrogen

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balance and deamination are discussed in the later section. Similar to nitrogen, phosphorus

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content in HTC process liquid is much higher than phosphorus content in untreated liquid


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septage. The phosphorus concentration, however, remains similar with HTC temperatures for

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shorter residence time. At longer residence times, phosphorus was increased for 180-120, while

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it is decreased for 230-120 and 260-120. As discussed earlier, phosphorus may have adsorbed

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onto hydrochar at higher temperatures resulting a lower phosphorus concentration in the HTC

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process liquid. Furhtermore, the basic conditions that occurred at higher reaction temperatures

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likely had a significant effect on phosphorous being found in lower amounts in the liquid phase,

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as was observed by Ekpo et al. with swine manure 21. Similar to nitrogen and phosphorus,

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potassium concentration increased with HTC treatment, which decreased with higher reaction

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temperatures and higher reaction times. This might be due to salt precipitations on the hydrochar

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pores, as chloride content in the HTC process liquid follows the similar trend as potassium.

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Among the micronutrients, calcium, magnesium, and iron concentrations were lower at

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higher HTC temperatures and were further decreased for HTC at 120 minutes compared to 30

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minutes at any given temperature. Unlike them, sodium, chloride, and sulfur concentrations were

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much higher at shorter residence time and increased with HTC temperature. However, the

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concentrations decreased with longer residence time. In general, monovalent anions and cations

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show more volatility with HTC temperatures and shorter residence times than divalent or

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polyvalent cations. However, the cation concentrations in bulk HTC process liquid were

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decreasing with prolonged residence time, except nitrogen compounds. For nitrogen, it seems

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like the deamination is more favorable than salt precipitations.

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3.4 HTC Solid Products as Fertilizers

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As the nutrients are concentrating on solid hydrochars with HTC treatments, it is

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important to determine the NPK fertilizer value of the hydrochars and how it changes with HTC

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conditions. Therefore, NPK fertilizer values were calculated based on nitrogen, phosphorus and 13 ACS Paragon Plus Environment


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potassium contents of the hydrochar and listed in Figure 1. In this case, phosphorus is shown as

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P2O5, whereas potassium is expressed as K2O, and nitrogen is considered elemental N. In

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general, raw solid septage contains high phosphorus (82 kg/tonne) and high nitrogen (29

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kg/tonne), but low potassium (4.5 kg/tonne). Therefore, it is understandable why raw septage is

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considered as solid fertilizer, especially in developing countries. From the discussion in the

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previous section, it is noticed that phosphorus is further concentrated in the hydrochar, especially

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at higher HTC temperatures and longer residence time. This reflects at the NPK fertilizer values

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as well, where phosphorus content reached as high as 130 kg/tonne for 260-120. However,

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nitrogen content is sacrificed in this process, where only about half of the nitrogen was remained

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in the 260-120 compared to raw septage. Although, potassium content increased with reaction

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time at higher HTC temperature, the overall effect of potassium in NPK remains very similar to

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raw septage. Therefore, it is reasonable to expressed that septage was originally nitrogen-rich

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fertilizer but converted to phosphorus-rich fertilizer with moderate amount of nitrogen on it.

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Now, compared to cow manure and hydrochars derived from cow manure, the septage

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hydrochars have less nitrogen (15 kg/tonne compared to 20 kg/tonne), but 5 to 10 times more

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phosphorus, and similar amounts of potassium 9.

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Septage contains moderate amount of phosphorus, which was concentrated with HTC.

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Moreover, HTC of septage yields large amount of NH4-N. It is also noticed that higher HTC

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temperature and reaction time, some indication of nutrient precipitation (magnesium, phosphorus

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etc.) occurred, especially at 260-120. The concentrations of nutrients are ideal for struvite

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precipitation. Struvite is a naturally occurring mineral fertilizer containing magnesium-

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ammonium-phosphate in hydrated form. Several researches were reported struvite precipitation

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from various wastes applying various technology other than HTC. It is therefore reasonable to


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test whether HTC of septage is creating struvite. To detect possible struvite precipitation, SEM

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and EDS imaging was taken of the 260-120 hydrochar. This hydrochar was selected due to

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having a low concentration of magnesium in the liquid phase. These images can be found in

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Figure 2. It is indeed seen that nitrogen, phosphorus, and magnesium are clumped together on

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hydrochar surface but distributed sparsely. In fact, the distribution of struvite compounds is

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rather uniform, though there are small crystal formations. This indicates that struvite

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precipitation has initiated but with an unfavorable pH (6 instead of 9-10) and perhaps a limiting

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quantity magnesium limits further crystallization of struvite crystals. A future study may warrant

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application of an appropriate crystallization environment to study struvite crystallization either

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in-situ or standalone process after HTC.

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3.5 HTC Process Liquid as Fertilizers

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While solid hydrochar produced from septage shows prominent potential for solid NPK

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fertilizer, HTC process liquid shows high concentration of NH4-N and micronutrients making it

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potential for liquid fertilizer. Moreover, a high COD content reflects a high humic content, which

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is beneficial for soils with less carbon and organic matters 22. Similar to marconutrients (NPK),

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which are beneficial for higher fertility of soils and plant growth, micronutrients also have

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significant importance in regard to the value of a liquid fertilizer. Magnesium concentrations

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decreases with treatment temperature and has no clear trend with respect to treatment time.

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Magnesium has been shown to be an important micronutrient in liquid fertilizers, and has even

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shown potential in reduce disease in potatoes 23. Similarly, calcium has also shown to be an

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important micronutrient in fertilizers and can help prevent mold 24. Calcium concentration in the

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liquid phase also decrease as treatment temperature increases with no clear trend with respect to

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reaction time. Sodium and chloride are the most abundant micronutrients found in the liquid 15 ACS Paragon Plus Environment


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products at all temperatures. Both are most abundant in the 260-30 product at 1900 mg/L for

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sodium and 1800 mg/L for chloride. Treatment for 120 minutes results in a near constant

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concentration of sodium and chloride around 1000-1100 mg/L. There was no significant

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difference between the treatment temperatures for 120 minutes. Only a trace amount of iron was

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found in the liquid product, but generally increased treatment temperature results in a lower

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amount of iron, with a sharp drop occurring between 220 and 260°C.

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3.6 Nitrogen Balance and Deamination

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In untreated septage, nitrogen is primarily found in the solid phase. The solid is 2.9%

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nitrogen by mass, while in the liquid phase nitrogen content is only at 19 mg/L. The nitrogen

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content is significantly lower than primary sewage sludge, which contains about 5.2% nitrogen

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14

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severity, HTC of septage results in a total decrease of nitrogen in the solid phase. Increased

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temperature results in lower amounts of solid nitrogen. 260-120 hydrochar contains 1.24%

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nitrogen, while 180-30 hydrochar contains 1.70% nitrogen. This is far less than hydrochars

337

produced for primary sewage, which have 4.1% nitrogen when treated at 180°C 30 minutes 14.

338

After HTC treatment, 62-72% of the nitrogen in the untreated septage is converted to soluble

339

species, with 180-30 liquid having the lowest conversion. Nitrate generally decreases with

340

temperature and time, and ammonia generally increases with temperature and time. Figure 3

341

shows the form the nitrogen species take in the liquid phase with each reaction condition. The

342

increased amount of ammonia with increasing reaction time is consistent with other

343

hydrothermal treatment studies for wastes and manures, such as with the treatment of chicken

344

manure at similar conditions 25. Soluble nitrogen as ammonia increases from 9% of total N in the

. However, unlike the primary sewage, where nitrogen concentrated in hydrochar with HTC


Page 17 of 30


345

180-30 sample to 53% in the 260-120 sample, while nitrate decreases from 19% to 8% in each

346

sample respectively.

347

The ratio of nitrogen as nitrate and ammonia changes drastically with different HTC

348

conditions, which can be seen in Table 2. Treatment at 180°C for 30 minutes produces the most

349

nitrate relative to ammonia, and treatment at 260°C for 120 minutes produces the least nitrate

350

relative to ammonia. This trend is similar to that found in HTC of chicken litter, in which longer

351

reaction times at higher temperatures produced more ammonia. Ammonia at the reported

352

concentrations can be removed and purified by air stripping 25. Additives can be used to both

353

adjust the pH and make use of the available ammonia though 26.

354

A possible mechanism for the formation of ammonia at higher temperatures and

355

treatment times is the deamination of amino acids found in the organic material present in the

356

septage feed. Deamination through hydrolysis would be consistent with the production of

357

carboxylic acids that result from hydrothermal treatment of organic compounds 27. Previous

358

research has shown that secondary amines will form ammonia or ammonium compounds when

359

hydrothermally treated at 200°C, which is also consistent with these findings 28.

360

3.7 Fate of Heavy Metals

361

HTC of septage concentrate NPK fertilizer in solid hydrochar and micronutrients in the

362

liquid phase. However, the applicability of septage-derived fertilizer products will depend on the

363

fate of heavy metals. Therefore, heavy metals are measured in both solid hydrochar and HTC

364

process liquid and listed in Table 2 and Table 3. All the heavy metals listed on the US EPA part

365

503 biosolids are measured. Only the listed heavy metals are found above detection limits (10

366

ppm). Among the listed heavy metals, copper and zinc are the highest for raw solid septage,

367

while none are detected on the liquid septage. Therefore, it is reasonable to state that heavy 17 ACS Paragon Plus Environment


Page 18 of 30

368

metals are predominantly stayed in the solid septage. Beside copper and zinc, strontium,

369

selenium, and nickel are found around 140-180 ppm, while chromium and lead were found

370

around 22 ppm and 51 ppm in the solid raw septage, respectively. Selenium content of the

371

untreated septage and of the product hydrochars were 10 times higher than the concentration of

372

selenium found in sewage sludge. The concentrations of the other tested heavy metals were

373

similar to those found in primary sewage sludge 29,30. The US EPA’s part 503 biosolids rule

374

limits the amount of heavy metals that are permissible in biosolid based fertilizers. According to

375

this rule, the permissible limit of heavy metals is 100 mg/kg of selenium, 840 mg/kg of lead,

376

4,300 mg/kg of copper, 420 mg/kg of nickel, 7,500 mg/kg of zinc, and 3,000 mg/kg of

377

chromium 31.

378

With the HTC, only selenium in the listed metals showed no significant change in

379

concentration in solid hydrochar. As a result, no selenium was found in the HTC process liquids.

380

Other heavy metals concentrations in the solid phase increases with both HTC reaction

381

temperatures. The extended reaction time further increases the concentration of heavy metals at

382

all temperatures. The single outlier of chromium in 220-120, but it is too close to the error range

383

also has a large standard deviation of ±15 mg/L. With the decrease of mass yield, the heavy

384

metals are concentrating in the solid phase, however, the concentration is found much higher

385

than only mass yield. For instance, zinc concentration in 260-30 is 61% more than raw septage,

386

while the mass loss is around 40%. The extra zinc might be adsorbed from bulk liquid, although

387

zinc is below detection level in the raw liquid septage. It has been reported previously that

388

hydrochar can adsorb trace heavy metals from the liquid, which is observed in this study as well

389

32

.


Page 19 of 30

390


Table 2 shows low amounts of heavy metals being found in the liquid phase, which

391

reflects the increase in concentration of the species in the solid phase as they stayed in the solid

392

phase while organic matter moved to the liquid phase during treatment. Arsenic, cobalt, and

393

mercury were also tested for, but were not found in concentrations above the detection limit of

394

the ICP method used (approximately 0.1 ppm). The immobility of the heavy metals in the solid

395

phase has been shown before in the subcritical treatment of sewage sludge 33.

396

Considering hydrochar application in soil, hydrochars produced at every HTC condition

397

meet all heavy metal requirements except selenium. All hydrochars are 30-40mg/kg above the

398

100 mg/kg limit for selenium. While the hydrochars have a good phosphorus number, the high

399

amount of selenium would limit their application as fertilizer in the US, unless they were mixed

400

in as a fertilizer additive to some other biosolid, such as cow manure. The EU has no restriction

401

on selenium in biosolids used for land application. All treated solids meet the current EU

402

standards for copper, lead, nickel, and zinc. Each nation within the EU can set more restrictive

403

standards though, just as states can in the US. The Netherlands has far more stringent

404

requirements for biosolids, with a limit of 75 ppm of copper and chromium compared to 1,750

405

and no limit for the EU 34. Since there is such a wide variety in requirements, application of

406

hydrothermally treated septage solids as fertilizer or biosolids would depend greatly on the

407

location within the EU.

408

In conclusion, behaviors of macro-and micro nutrients of septage differ with HTC

409

process conditions. Despite of decrease of carbon in hydrochar with HTC severity, phosphorus is

410

increased significantly at 260-120. Furthermore, more deamination is observed in the HTC

411

process liquid for 260-30 and 260-120, where ammonia was predominantly formed over nitrate.

412

Monovalent cations and anions and divalent cations are increased in hydrochar at longer 19 ACS Paragon Plus Environment


Page 20 of 30

413

residence time at 260°C. In terms of NPK fertilizer, hydrochar is promising as fertilizers, with

414

high P values. Except selenium, concentrations of others heavy metals in hydrochar are in

415

acceptable range for soil applications.

416

4. Acknowledgements

417

M. Toufiq Reza acknowledges start-up funding support from the Ohio University. The authors

418

also acknowledge Dr. Wen Fan for laboratory support. The authors are also thankful to Mr.

419

Pretom Saha, Mr. Akbar Saba, and Mr. Md Rifat Hasan at the Institute for Sustainable Energy

420

and the Environment (ISEE) for their laboratory efforts on this project. The authors would like to

421

thank Mr. Mike Cooper from the Athens City-County Health Department for providing septage

422

along with his valuable assistance.

423

References

424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448

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Massoud, M. A.; Tarhini, A.; Nasr, J. A. Decentralized approaches to wastewater treatment and management: Applicability in developing countries. J. Environ. Manage. 2009, 90 (1), 652–659. USEPA. Decentralized Wastewater Management Program Highlights; EPA-832-R-140006; USEPA, 2014. Clasen, T. F.; Bostoen, K.; Schmidt, W.-P.; Boisson, S.; Fung, I. C.-H.; Jenkins, M. W.; Scott, B.; Sugden, S.; Cairncross, S. Interventions to improve disposal of human excreta for preventing diarrhoea. Cochrane Libr. 2010. Childers, D. L.; Corman, J.; Edwards, M.; Elser, J. J. Sustainability challenges of phosphorus and food: solutions from closing the human phosphorus cycle. Bioscience 2011, 61 (2), 117–124. Jakob, A.; Stucki, S.; Struis, R. P. W. J. Complete heavy metal removal from fly ash by heat treatment: influence of chlorides on evaporation rates. Environ. Sci. Technol. 1996, 30 (11), 3275– 3283. Busch, D.; Kammann, C.; Grünhage, L.; Müller, C. Simple biotoxicity tests for evaluation of carbonaceous soil additives: establishment and reproducibility of four test procedures. J. Environ. Qual. 2012, 41 (4), 1023–1032. Heilmann, S. M.; Molde, J. S.; Timler, J. G.; Wood, B. M.; Mikula, A. L.; Vozhdayev, G. V.; Colosky, E. C.; Spokas, K. A.; Valentas, K. J. Phosphorus reclamation through hydrothermal carbonization of animal manures. Environ. Sci. Technol. 2014, 48 (17), 10323–10329. Jambaldorj, G.; Takahashi, M.; Yoshikawa, K. Liquid fertilizer production from sewage sludge by hydrothermal treatment. In Proceedings of International Symposium on EcoTopia Science; 2007. Toufiq Reza, M.; Freitas, A.; Yang, X.; Hiibel, S.; Lin, H.; Coronella, C. J. Hydrothermal carbonization (HTC) of cow manure: carbon and nitrogen distributions in HTC products. Environ. Prog. Sustain. Energy 2016, 35 (4), 1002–1011. Zhang, G.; Ma, D.; Peng, C.; Liu, X.; Xu, G. Process characteristics of hydrothermal treatment of antibiotic residue for solid biofuel. Chem. Eng. J. 2014, 252, 230–238. 20 ACS Paragon Plus Environment

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Zhuang, X.; Huang, Y.; Song, Y.; Zhan, H.; Yin, X.; Wu, C. The transformation pathways of nitrogen in sewage sludge during hydrothermal treatment. Bioresour. Technol. 2017, 245, 463– 470. Ekpo, U.; Ross, A. B.; Camargo-Valero, M. A.; Williams, P. T. A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate. Bioresour. Technol. 2016, 200, 951–960. Liu, X.; Li, Z.; Zhang, Y.; Feng, R.; Mahmood, I. B. Characterization of human manure-derived biochar and energy-balance analysis of slow pyrolysis process. Waste Manag. 2014, 34 (9), 1619– 1626. Danso-Boateng, E.; Shama, G.; Wheatley, A. D.; Martin, S. J.; Holdich, R. G. Hydrothermal carbonisation of sewage sludge: effect of process conditions on product characteristics and methane production. Bioresour. Technol. 2015, 177, 318–327. Markus, J.; McBratney, A. B. A review of the contamination of soil with lead: II. Spatial distribution and risk assessment of soil lead. Environ. Int. 2001, 27 (5), 399–411. Pais, I.; Jones Jr, J. B. The handbook of trace elements; CRC Press, 1997. Funke, A.; Ziegler, F. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuels Bioprod. Biorefining 2010, 4 (2), 160– 177. Escala, M.; Graber, A.; Junge, R.; Koller, C. H.; Guiné, V.; Krebs, R. Hydrothermal carbonization of organic material with low dry matter content: the example of waste whey. J. Residuals Sci. Technol. 2013, 10 (4). Escala, M.; Zumbuhl, T.; Koller, C.; Junge, R.; Krebs, R. Hydrothermal carbonization as an energy-efficient alternative to established drying technologies for sewage sludge: a feasibility study on a laboratory scale. Energy Fuels 2012, 27 (1), 454–460. Reza, M. T.; Nover, J.; Wirth, B.; Coronella, C. J. Hydrothermal carbonization of glucose in saline solution: sequestration of nutrients on carbonaceous materials. 2016. Ekpo, U.; Ross, A. B.; Camargo-Valero, M. A.; Fletcher, L. A. Influence of pH on hydrothermal treatment of swine manure: Impact on extraction of nitrogen and phosphorus in process water. Bioresour. Technol. 2016, 214, 637–644. Edmeades, D. C. The long-term effects of manures and fertilisers on soil productivity and quality: a review. Nutr. Cycl. Agroecosystems 2003, 66 (2), 165–180. Klikocka, H. Influence of NPK fertilization enriched with S, Mg, and micronutrients contained in liquid fertilizer Insol 7 on potato tubers yield [Solanum tuberosum L.] and infestation of tubers with Streptomyces scabies and Rhizoctonia solani. J. Elem. 2009, 14 (2), 271–288. Volpin, H.; Elad, Y. Influence of calcium nutrition on susceptibility of rose flowers to Botrytis blight. Phytopathology 1991, 81 (11), 1390–1394. Huang, W.; Yuan, T.; Zhao, Z.; Yang, X.; Huang, W.; Zhang, Z.; Lei, Z. Coupling Hydrothermal Treatment with Stripping Technology for Fast Ammonia Release and Effective Nitrogen Recovery from Chicken Manure. ACS Sustain. Chem. Eng. 2016, 4 (7), 3704–3711. Rahman, M. M.; Salleh, M. A. M.; Rashid, U.; Ahsan, A.; Hossain, M. M.; Ra, C. S. Production of slow release crystal fertilizer from wastewaters through struvite crystallization–A review. Arab. J. Chem. 2014, 7 (1), 139–155. Quitain, A. T.; Faisal, M.; Kang, K.; Daimon, H.; Fujie, K. Low-molecular-weight carboxylic acids produced from hydrothermal treatment of organic wastes. J. Hazard. Mater. 2002, 93 (2), 209–220. He, C.; Zhao, J.; Yang, Y.; Wang, J.-Y. Multiscale characteristics dynamics of hydrochar from hydrothermal conversion of sewage sludge under sub-and near-critical water. Bioresour. Technol. 2016, 211, 486–493. Bradford, G. R.; Page, A. L.; Lund, L. J.; Olmstead, W. Trace element concentrations of sewage treatment plant effluents and sludges; their interactions with soils and uptake by plants. J. Environ. Qual. 1975, 4 (1), 123–127. 21 ACS Paragon Plus Environment


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Furr, A. K.; Lawrence, A. W.; Tong, S. S.; Grandolfo, M. C.; Hofstader, R. A.; Bache, C. A.; Gutenmann, W. H.; Lisk, D. J. Multielement and chlorinated hydrocarbon analysis of municipal sewage sludges of American cities. Environ. Sci. Technol. 1976, 10 (7), 683–687. Walker, J.; Knight, L.; Stein, L. Plain english guide to the EPA part 503 biosolids rule. In Plain english guide to the EPA part 503 biosolids rule; EPA, 1994. Regmi, P.; Moscoso, J. L. G.; Kumar, S.; Cao, X.; Mao, J.; Schafran, G. Removal of copper and cadmium from aqueous solution using switchgrass biochar produced via hydrothermal carbonization process. J. Environ. Manage. 2012, 109, 61–69. Shi, W.; Liu, C.; Ding, D.; Lei, Z.; Yang, Y.; Feng, C.; Zhang, Z. Immobilization of heavy metals in sewage sludge by using subcritical water technology. Bioresour. Technol. 2013, 137, 18–24. Iranpour, R.; Cox, H. H. J.; Kearney, R. J.; Clark, J. H.; Pincince, A. B.; Daigger, G. T. Regulations for biosolids land application in US and European Union. J. Residuals Sci. Technol. 2004, 1 (4), 209–22.

514 515 516 517 518


Page 23 of 30


519

Table and Figures List:

520

Table 1. Characterization of Hydrochar Samples and Untreated Septage. All values are shown as

521 522

mass%. Table 2. Macronutrients, micronutrients, and heavy metals in the HTC Liquid Products and

523

Untreated Liquid. Units in ppm, ~ denotes below detection limit.

524

Table 3. Heavy Metals Distributions in Hydrochars and Untreated Solids.

525 526 527 528 529 530

Figure 1. NPK of Hydrochars and Untreated Solid Septage. N as Nitrogen, P as P2O5, K as K2O. Units in kg/tonne. Figure 2. SEM image of septage-treated hydrochar at 260-120 (above), and P, N, and Mg elemental mapping of 260-120 hydrochar (below) Figure 3. Nitrogen distribution in liquid phase at various HTC conditions

531 532 533 534 535 536 537 538 539 540



Page 24 of 30

Table 1. Characterization of Hydrochar Samples and Untreated Septage. All values are shown as mass%.

541 Temperature (°C) Untreated 180 220 260 180 220 260

Time (minutes)

30

120

Mass Yield 67±1 65±1 60±1 68±1 66±1 60±1

Ash

C%

H%

N%

P%

Ca%

Fe%

K%

Mg%

Na%

S%

46±2 50±1 57±1 60±1 53±1 57±1 64±1

30.9±0.3 27.9±0.7 28.8±0.4 25.4±0.9 27.6±0.7 26.5±0.4 25.2±0.5

4.7±0.1 4.4±0.2 4.3±0.1 3.6±0.2 4.3±0.2 3.4±0.1 3.5±0.2

2.90±0.02 1.70±0.10 1.32±0.06 1.24±0.01 1.53±0.01 1.24±0.04 1.40±0.08

3.72±0.02 4.42±0.07 5.0±0.1 5.5±0.1 4.60±0.03 4.97±0.02 5.67±0.08

3.22±0.02 4.0±0.1 4.17±0.07 4.8±0.3 3.78±0.04 3.75±0.07 4.30±0.01

0.66±0.01 0.79±0.01 0.87±0.01 1.01±0.02 0.83±0.02 0.92±0.06 1.05±0.01

0.39±0.01 0.28±0.04 0.27±0.04 0.47±0.04 0.22±0.02 0.23±0.02 0.35±0.01

0.47±0.02 0.6±0.1 0.62±0.03 0.85±0.05 0.51±0.02 0.59±0.05 0.70±0.01

1.28±0.05 0.2±0.1 0.2±0.1 0.41±0.08 0.11±0.04 0.11±0.01 0.21±0.05

1.46±0.03 0.91±0.04 0.87±0.04 1.0±0.1 0.91±0.02 0.91±0.02 0.75±0.03

542 543 544 545 546 547 548 549 550

Table 2. Macronutrients, micronutrients, and heavy metals in the HTC Liquid Products and Untreated Liquid. Units in ppm, ~ denotes

551

below detection limit. 24 ACS Paragon Plus Environment

Page 25 of 30


552 553

Micronutrients

Macronutrients

Heavy Metals

COD (g O2/L)

pH

P (mg/L)

NO3(mg/L)

NH4+ (mg/L)

N (mg/L)

K (mg/L)

Ca (mg/L)

Na (mg/L)

Mg (mg/L)

Fe (mg/L)

S (mg/L)

Cl (mg/L)

Ni (mg/L)

Cu (mg/L)

Sr (mg/L)

Zn (mg/L)

0.150±0.050

9

1.2±0.2

10.0±0.5

18±1

19±1

~

9.4±0.9

1.5±1

1.5±0.2

~

~

12±2

~

~

~

~

26.9±0.3

6.5

49±3

1700±200

230±20

1800±200

260±30

130±20

1300±100

58±3

6.6±0.3

780±80

1200±100

1.0±0.1

0.21±0.09

0.35±0.03

0.45±0.09

30.1±0.2

6.5

42±1

860±10

310±20

2100±200

200±30

100±20

1700±300

40±1

7.2±0.5

570±50

1800±200

1.1±0.1

0.10±0.04

0.28±0.05

0.31±0.05

260

30.5±0.5

8

48±7

1430±50

780±60

2100±100

290±20

53±5

1900±300

11±1

2.0±0.1

770±80

1800±200

0.2±0.1

0.13±0.01

0.21±0.04

0.17±0.04

180

26.6±0.3

6.5

58±6

1200±100

590±40

1850±50

151±8

160±10

1100±100

52±4

8.2±0.8

490±30

1100±100

1.6±0.1

0.10±0.01

0.39±0.07

0.38±0.05

27.7±0.2

7

37±5

740±50

980±70

2000±100

134±3

100±10

970±50

25±4

3.7±0.3

440±40

1000±100

0.8±0.1

0.06±0.01

0.27±0.09

0.28±0.09

27.9±0.5

8

45±4

760±80

1430±20

2100±50

124±4

37±1

1100±100

12±2

1.5±0.3

430±50

1000±100

~

~

0.21±0.02

0.10±0.01

Temperature (°C)

Time (minutes)

Untreated

180

220

220 260

30

120



554

Page 26 of 30

Table 3. Heavy Metals Distributions in Hydrochars and Untreated Solids. Temperature Time Cr Cu Ni Pb Se Sr Zn (°C) (minutes) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) Untreated 22±5 410±5 140±10 51±4 142±5 180±5 420±20 180-30 38±10 520±7 150±10 71±3 125±8 214±7 530±10 30 220-30 45±9 590±7 170±30 79±3 130±10 243±3 680±20 260-30 60±10 610±20 210±50 86±2 126±3 260±10 690±20 180-120 48±6 550±10 150±30 77±4 138±4 221±4 560±10 120 220-120 70±15 600±3 220±50 78±4 130±10 233±7 620±10 260-120 62±9 630±30 240±5 89±5 134±8 256±8 680±20

555 556 557 558 559


Page 27 of 30


560 140.0

120.0

kg/tonne

100.0

80.0

60.0

40.0

20.0

0.0 Untreated

180-30

180-120

220-30 N

P

220-120

260-30

260-120

K

561 562 563

Figure 1. NPK of Hydrochars and Untreated Solid Septage. N as Nitrogen, P as P2O5, K as K2O. Units in kg/tonne.

564 565 566 567 568 569 570 571 572 573 574 27 ACS Paragon Plus Environment


Page 28 of 30

575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599

Figure 2. SEM image of septage-treated hydrochar at 260-120 (above), and P, N, and Mg elemental mapping of 260-120 hydrochar (below)


Page 29 of 30


100% 90% 80% 70%

%N

60%

% Other N

50%

% Nitrate

40%

% Ammonia

30% 20% 10% 0%

600

180-30

180-120

220-30

220-120

260-30

260-120

601 602

Figure 3. Nitrogen distribution in liquid phase at various HTC conditions

603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 29 ACS Paragon Plus Environment


620

Page 30 of 30

TOC Graphic

621 622 623 624 625 626 627 628 629 630 631

TOC graphic: Hazardous septic tank waste can be hydrothermally treated to produce useful solid and liquid products that are promising fertilizers.


Recovery of Macro and Micro-Nutrients by Hydrothermal

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