Comparison of Adsorptive and Biological Total Organic Carbon

12

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch012

Comparison of Adsorptive and Biological Total Organic Carbon Removal by Granular Activated Carbon in Potable Water Treatment S. W. M A L O N E Y , K. BANCROFT, and I. H. S U F F E T Drexel University, Environmental Studies Institute, Philadelphia, PA 19104

P. R. CAIRO Philadelphia Water Department, Research and Development, Philadelphia, PA 19107

The mechanism of long-term quasi-steady-state removal of total organic carbon by granular activated carbon is explained in terms of two current theories: biological total organic carbon removal and slow adsorption kinetics. Eight parallel columns indicated that granular activated-carbon contactors are reproducible for total organic carbon, dissolved oxygen, pH, and alkalinity changes across the column when conditions are identical. Carbon regeneration may be assisted by bacteria but is not caused by bacteria. Temperature and total organic carbon removal were not related over a short time, but temperature and dissolved oxygen removal were strongly related.

R

ECENT ADVANCES IN ANALYTICAL CHEMISTRY led to the discovery of

nanogram per liter to microgram per liter concentrations of undesirable and potentially harmful low molecular weight nonpolar organic chemicals in raw water supplies and finished drinking water. One viable technique for the removal of a broad spectrum of organic chemicals is activated-carbon adsorption. Granular activated carbon (GAC) provides a vast surface area for adsorption of nonpolar organic chemicals. The major drawback of activated carbon is high operating costs associated with regeneration of spent carbon.

0065-2393/83/0202-0279$07.00/0 © 1983 American Chemical Society

In Treatment of Water by Granular Activated Carbon; McGuire, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1983.


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T R E A T M E N T O F W A T E R BY G R A N U L A R A C T I V A T E D C A R B O N

The purpose of the G A C is to remove trace organics of health and organoleptic concern. These trace organic compounds are only a small portion of the total organic carbon (TOC), so monitoring T O C removal does not give a measure of health-related trace organic removal. However, comparison of single solute adsorption isotherms in the presence and absence of natural background organics (I) showed that competition for adsorption sites may occur between natural organics in water and organics of health concern. Therefore, reducing the load of natural T O C onto the GAC may extend the useful life of the GAC for removal of health-related organics. Many researchers investigating G A C are studying mechanisms that may increase the carbons useful life. One parameter often used to evaluate carbon contactor performance is T O C removal Typically, GAC contactors exhibit a large initial capacity for T O C followed by partial breakthrough to a level of quasi-steady-state removal (2,3). The objective of the research reported here was to develop an understanding of the mechanism of this long-term quasi-steady-state removal. Two current theories used to explain the long-term removal are biological T O C removal (4-6) and slow adsorption kinetics (7 8). The use of microorganisms on the G A C to help remove T O C is of particular interest as is increasing the biodegradability of naturally occurring T O C which appears to occur by pre-ozonation (9). f

Biological Regeneration The occurrence of large numbers of bacteria on GAC filters is well documented (2). In chlorinated treatment plants, bacterial densities are much higher in the effluent than in the influent of a carbon contactor. This increase has been attributed to removal of chlorine and other toxic materials by the top of the G A C column, concentration of organics on the carbon surface, adsorption of bacteria by GAC, and a "sheltered" environment in the macropores of the G A C which shields bacteria from shear forces. These same factors apply in a nonchlorinated system. Enhanced biodegradation of organics on G A C is proposed to occur due to acclimation of the microbes to the adsorbed substrate (5). In this proposed complex process, GAC adsorbs recalcitrant organics as well as bacteria. Thus, the carbon keeps the bacteria in contact with higher concentrations of refractory organics than in the bulk liquid. Maintaining contact between the bacteria and adsorbed recalcitrant organics allows the bacteria to acclimate to the recalcitrant organics. Biological regeneration emphasizes that the solute must be adsorbed first (4, 6). The bacteria then use a portion of the retained organics, opening sites on the GAC for further adsorption of other organics.



12. MALONEY ET AL. Adsorptive and Biological TOC Removal by GAC281 The action of GAC in adsorbing organics decreases the aqueous phase concentration of organics, and, logically, this change would inhibit acclimation of bacteria to adsorbed substrate. Thus, acclimation has been referred to as a common fallacy in activated-carbon adsorption theory (10). Typically, in a real situation, the influent concentration of organic compounds is variable. Some of the time, carbon is adsorbing the organic in question, but when the influent concentration drops to a low or zero value, the GAC may release the organic. In this case, carbon acts as a buffer for bulk phase concentration of an organic that has a rapidly varying influent concentration, and acclimation of bacteria is possible on GAC; on an inert substance such as sand, acclimation is less likely due to the sporadic appearance and disappearance of the organic in the aqueous phase. Improved biodegradability of naturally occurring T O C has been observed following ozonation (9). Therefore, the interaction of ozonation and GAC unit processes may be engineered to maximize the long-term T O C removal capability. It is proposed that carbon bedlife is prolonged by converting a portion of the recalcitrant organics to biodegradable organics in ozonation (4). The attached microbes then convert the biodegradable portion to biomass, carbon dioxide, and waste products. The T O C that is converted to biomass or carbon dioxide does not take space on the GAC. Other biological end products may or may not adsorb, but the net result is a lesser load to the GAC.

Slow Adsorption Kinetics The long-term removal of T O C by GAC also can be explained by a dual rate kinetic model (7,8). Two distinct stages of adsorption have been observed during batch equilibrium studies. In the first stage, rapid adsorption to 50%-80% of the total carbon capacity is observed within a few hours. The remaining adsorptive capacity is exerted very slowly. The dual rate kinetic model takes both stages into account and predicts the quasi-steady-state removal so commonly seen on GAC for lumped parameters such as TOC. This model of activated carbon considers, as a first approximation, two types of pores. Macropores are defined as those pores in which diffusion rates are unhindered by the pore walls, and micropores are those pores with radii of comparable size to the diffusing species (7). Because of strong multidirectional adsorptive forces in micropores and the restricted diffusion rates caused by the proximity of the pore walls, transport rates are considerably lower than in the large pores. The rapid initial uptake rate in batch isotherms and large initial removals encountered in column studies are assumed to occur in the macropores. The slow approach to equilibrium


282


after initial uptake observed in batch studies and the quasi-steady-state removal seen in column studies are attributed to the micropores.


Experimental A method for differentiating between adsorptive and biological removal was required in which little modification of the actual operation of a carbon contactor occurred. The method chosen was temperature control of the contactor influent. By using this method, the contactors were operated at actual field conditions, because the temperatures were controlled within the temperature range normally encountered at the water treatment plant. Furthermore, the contactors were subjected to varying influent temperatures such as encountered in a full-scale facility. A temperature differential was maintained between the contactors by chilling the influent to the cold contactor during the summer and early fall and by heating the influent to the warm contactor during the late fall and winter. Figure 1 shows expected results. The difference between the warm and cold effluent gives an estimate of the bacterial contribution. The temperature dependency of most reactions is represented by Arrhenius' law (11):

k = k e- " Q

E

,RT

U)

where k is reaction rate at temperature T, E is the activation energy, R is the universal gas law constant, and T is the absolute temperature. The rate of the removal reaction determines the magnitude of the removal as the reaction is limited by the contact time. The overall removal rate is actually the sum of many individual rates for various compounds that make up the T O C . The key to the reaction rate is E . Physical mechanisms such as diffusion and adsorption have small activation energies, up to 5 kcal/mol K. These reactions show little temperature dependency. Previous work with activated carbon indicates that normal temperature variations have only minor effects on adsorption a

a

(12).

For a 10°C temperature increase, a general rule of thumb for biological systems states that the reaction rate should double. This rate corresponds to an activation energy of 10-20 kcal/mol K. Stephenson (13) measured the effect of temperature on Escherichia coli using various single substrates and found an average ratio of substrate utilization rates of 2 for the 2 3 ° C - 3 3 ° C range in a growth limited system. In an (activated sludge) substrate limited system, the ratio of rates has been measured as low as 1.35 (14). In a study similar to the one reported here, parallel G A C contactors treating wastewater effluent were operated at different temperatures (5°C and 25°C). The results of that study (15) indicate that temperature can be used to control bioactivity on G A C as the warm contactor exhibited greater removal capacity for T O C than the cold contactor. Average influent T O C concentration was 20 mg/L for the carbon contactors. In this parallel column study, all columns were receiving an identical influent. The objective of starting all columns on ambient influent was to develop a bacterial population acclimated to warm temperatures and then "shock" the bacteria on one set of columns with a sudden temperature change. This procedure avoids the possibility of selecting for cold-adapted microorganisms in the cold columns.



12.

MALONEY ET AL.

Adsorptive and Biological TOC Removal by GAC

283

Biological activity was monitored by substrate utilization and the modified plate count (16) on the G A C and in the influent and effluent. Organic removal was monitored by T O C , rriacroreticular resin (MRR) extraction, and carbon core analysis. This paper deals only with temperature effects on T O C and dissolved oxygen (DO) removal in the carbon adsorption process. D O removal was monitored with an Orion Probe (#97-08). T O C was monitored on a Dohrman 54-D low level analyzer. Alkalinity and p H were monitored for 13 weeks. However, no change was observed in the pH or alkalinity across the contactor (7 weeks at ambient temperature, 6 weeks in ambient-chilled mode) even though dramatic effects were observed in the D O removal when the temperature was lowered in two columns (after 7 weeks). D O was sampled in a standard B O D bottle and was allowed to overflow for a sufficient time to replace the bottle volume twice. This method of overflowing was employed because the chilled sample appeared to be picking up oxygen from the air and consistently had a higher D O than the ambient sample. After the overflowing technique was adopted, the influents to the cold and warm columns registered the same D O concentration, indicating that absorption of oxygen from the atmosphere was no longer a problem. Pilot Plant Facility. The source of influent water was the pilot plant at the Philadelphia Torresdale water treatment plant located on the Delaware River estuary in northeast Philadelphia The complete pilot plant has been described elsewhere (17). Figure 2 shows a schematic of the pilot plant. In general, raw

TIME

Figure 1. Conceptual diagram of total organic carbon (TOC) removal as a function of temperature. Reading the curves from top to bottom: influent, cold effluent, and warm effluent.




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12.

MALONEY ET AL.

Adsorptive and Biological TOC Removal by GAC 285

water is pumped from the Delaware River into a raw water basin, then to an upflow clarifier with chemical addition (FeCl and NaOH) en route from the raw water basin to the clarifier. Water from the clarifier flows through a rapid sand filter into backwash tanks. The pilot plant water is not disinfected to this point. Water from the backwash tanks is pumped through an ozone contactor into a retention tank which allows for dissipation of ozone residual. Minicolumn Operational Characteristics. Water from the ozone retention tank is pumped through a small sand filter to eight parallel G A C minicolumns. Figure 3 shows a schematic of the minicolumn apparatus and Table I gives its physical characteristics. Table II summarizes the modes of minicolumn operation. For the first 7 weeks of the experiment, all columns received ambient influent (Mode 1). During this period, Columns 1-4 were taken offline at 2-week intervals, and carbon samples were analyzed for bacterial densities and chemical analyses (not reported on in this paper). At the beginning of Week 8, the influent temperature to two of the remaining four columns was reduced by passing water through a cooling bath in 6.1 m (20 ft) of 0.6-cm (^-in.) stainless steel tube (Mode 2). Two remaining columns (one cold and one warm) were taken offline 2 weeks later and analyzed as described.


3

OZONE RETENTION TANK

SAND FILTER DETENTION TIME < 1 MINUTE

COOLING BATH

G A C

G A C

G A C

G A C

G A C

G A C

3

4

5

6

G A C

Figure 3. Scheme of minicolumn.


AFTER SEVEN WEEKS

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Table I. Minicolumn F Diameter Length Material Flow rate EBCT Carbon type Carbon depth Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch012

a

sical Characteristics 4 cm (1.6 inches) 102 cm ( 4 0 ± inches) Teflon 80 mL/min 15 min Filtrasorb 400 91 cm ( 3 6 ± inches)

Empty Bed Contact Time.

The original intent of the study was to observe differences in T O C removal after bacterial populations had been established. No difference in T O C removal as a result of temperature control was observed during Mode 2 (Table II). This finding led to two possible conclusions: 1. The bacterial contribution to T O C removal in an activated-carbon contactor was too small (

Comparison of Adsorptive and Biological Total Organic Carbon

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