Ind. Eng. Chem. Res. 2007, 46, 6661-6665
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Aerobic Granular Sludge Systems: The New Generation of Wastewater Treatment Technologies C. Di Iaconi,*,† R. Ramadori,‡ A. Lopez,† and R. Passino‡ Istituto di Ricerca Sulle Acque C.N.R., Via F. De Blasio 5, 70123 Bari, Italy, and Istituto di Ricerca Sulle Acque C.N.R., Via Reno 1, 00198 Roma, Italy
The paper reports the results of a laboratory investigation aimed at evaluating the effectiveness of an innovative technology, SBBGR (sequencing batch biofilter granular reactor), based on aerobic granular biomass, for treating diluted (i.e., municipal wastewater) or concentrated (i.e., municipal landfill leachates) wastewater. When this technology was applied to the treatment of municipal wastewater, the results showed that, even at maximum organic load (i.e., 7 (kg of COD)/m3‚d), the chemical oxygen demand (COD) in the treated effluent was lower than 50 mg/L. In addition, total Kjeldahl nitrogen (TKN) removal efficiency was higher than 87% up to an organic load of 5.7 (kg of COD)/m3‚d, corresponding to a nitrogen load of 0.8 (kg of TKN)/m3‚d. During the treatment of a mature municipal landfill leachate, the SBBGR proved suitable for removing the entire biodegradable compound content (i.e., about 80% of the COD content of the leachate) up to an applied organic loading value of 1.1 (kg of COD)/m3‚d. During the whole investigation, the process was characterized by a low sludge production, about 1 order of magnitude lower than that of conventional systems. 1. Introduction The two major problems in the field of wastewater biological treatments recognized worldwide are the production of huge amounts of sludge, usually associated with the treatment of concentrated wastewater, and the difficulty in treating diluted wastewater (such as municipal effluents) in which a very low biomass concentration is achieved in conventional technologies (i.e., activated sludge systems) with negative repercussions on conversion capacities and consequent large area requirements. This explains the great effort being made worldwide to find a replacement for conventional technologies, with innovative ones aimed at greater compactness, better operational flexibility, and lower sludge production.1 Among the novel biological technologies recently developed for wastewater treatment, the most attractive are systems based on aerobic granular biomass. In such systems, the biomass grows as compact and dense microbial granules, allowing greater biomass retention in the reactor to be obtained, with interesting repercussions on substrate conversion capacities. In fact, biomass concentrations as high as 6.0-12 g/L and applied organic loading rates up to 6-8 (kg of COD)/m3‚d are commonly reported in the literature.2 Aerobic granular sludge is a young technology, and many aspects still need to be investigated. For example, the causes and mechanism of aerobic granulation are not yet fully understood, although several factors have been described in the literature, which may play an important role.3-11 In particular, the operational periodic conditions seem to be crucial for the granular growth. In fact, so far, aerobic granulation has been observed mainly in suspended biomass-sequential batch reactors (SBRs).12-15 With a different approach, the Water Research Institute (IRSA) of the Italian National Research Council (CNR) has been developing an aerobic granular biomass system, known with * Correspondingauthor.Phone: +390805820511.Fax: +390805313365. E-mail:
[email protected]. † Istituto di Ricerca Sulle Acque C.N.R.-Bari. ‡ Istituto di Ricerca Sulle Acque C.N.R.-Roma.
the acronym SBBGR (sequencing batch biofilter granular reactor), based on a submerged biofilter that operates in a “fill and draw” mode. In this system, the granules are entrapped in the pores produced by packing of bed material allowing greater biomass retention in the reactor to be obtained (up to 50 g/L) with interesting repercussions on substrate conversion capacities. In addition, SBBGR technology is characterized by a very low sludge production.16 In the SBBGR system, the granulation process takes place during the reactor start-up period.17,18 Four steps can be distinguished in this process: the formation of a thin biofilm that completely covers the carrier surface; an increase in biofilm thickness; the break-up of attached biofilm with the release of biofilm particles; and the rearrangement of biofilm particles in smooth granules. The present paper reports the results of an investigation aimed at assessing the effectiveness of SBBGR technology for treating both municipal wastewater, characterized by low concentrations of pollutants, and municipal landfill leachates, chosen as representative of concentrated wastewater. 2. Materials and Methods 2.1. SBBGR System and Operative Schedule. The investigation was carried out using a lab-scale SBBGR, as shown in Figure 1. It consisted of a closed cylindrical plexiglass vessel (geometric volume ) 27 L; working liquid volume ) 12 L) filled (fixed bed volume ) 15 L) with biomass support material [plastic element (see Figure 1), length ) 7 mm; diameter ) 10 mm; specific surface ) 630 m2/m3; density ) 0.95 g/cm3; bed porosity ) 0.75] held between two sieves and aerated by air injection through porous stones placed close to the upper sieve. An external loop allowed wastewater recirculation through the bed, by means of a pump, in order to obtain a homogeneous distribution of substrate and oxygen. A pressure meter (manostat), set in the bottom of the reactor, measured biofilter head losses on-line. When a fixed set value of head loss was reached, a washing step was carried out using compressed air (1.5 bar) in order to remove the excess biomass. The removed biomass was collected and measured in order to
10.1021/ie061662l CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007
Figure 1. Sketch (a) and photo (b) of SBBGR. Table 1. Operative Conditions of SBBGR Throughout the Experimentation period A subperiod parameter duration (months) organic load ((kg of COD)/m3·d) nitrogen load ((kg of N)/ m3·d) hydraulic residence time (d) Cycle Phase Duration (min) filling aerobic degradation drawing phase
1 1.5 0.9 0.12 0.4
period B subperiod
2
3
4
5
6
7
8
9
10
1.5 1.8 0.25 0.3
1.5 3.6 0.6 0.25
1.5 5.7 0.8 0.16
1.5 7.0 1.1 0.12
1.5 0.6 0.004 26
1.5 1.1 0.006 15
1.5 2.2 0.012 9
1.5 3.4 0.018 5
1.5 4.5 0.024 4
10 220-700 10
calculate the specific sludge production. Parameters such as dissolved oxygen (DO), temperature, and pH were monitored on-line. The system was fully automatic. A high air-flow rate value (i.e., 200 L/h) was used, in order to keep the dissolved oxygen concentration above the bed close to the saturation value. This allowed the dissolved oxygen concentration throughout the bed to be kept in the range 5-7 mg/ L, except during the filling time, when DO was lower. Furthermore, the air-flow rate was controlled by a flow meter; the temperature of the reactor was maintained at 20 °C using a water jacket and a thermostat bath. The pH, which was continuously monitored, remained in the range 7.0-7.5. The operative schedule of the plant is reported in Table 1. In practice, after a 3-month start-up period, during which careful attention was paid to the generation of granular biomass,16 the SBBGR was operated for an additional 15 months, divided into two periods (A and B) of experimental activities. During period A (length ) 7.5 months), the SBBGR treated the primary effluent coming from a municipal wastewater treatment plant, while in period B (length ) 7.5 months), it was fed with a mature leachate coming from a local municipal landfill that had been pretreated to reduce the high ammonia content by forming ammonium phosphate. Periods A and B were split into five subperiods, each characterized by progressively increasing organic loads (see Table 1) in order to assess the maximum value of the organic load compatible with good chemical oxygen demand (COD) and nitrogen and total suspended solids (TSS) removal efficiencies. In both periods, the SBBGR worked with a cycle organized in three consecutive phases, namely, filling, aerobic degradation, and drawing phases. 2.2. Analytical Methods. COD (chemical oxygen demand), TKN (total Kjeldahl nitrogen), NH4-N (ammoniacal nitrogen),
10 1420 10
NO3-N (nitrate nitrogen), PO4-P (orthophosphate-phosphorus), TSS (total suspended solids), and VSS (volatile suspended solids) were determined according to standard methods.19 Instantaneous and composite samples of influent and effluent were taken using automatic samplers. Oxygen consumption (OC) of biomass was measured using a thermostatted closed batch reactor (1 L working volume) characterized by a high liquid/gas volume ratio (VL/VG ) 100). The content of the reactor was thoroughly stirred by a magnetic mixer. There were three openings in the reactor cover, one for inserting the oxygen electrode (WTW OXI cell 325), one for the aeration frit, and one for sampling. The oxygen electrode was interfaced with a PC for automatic recording of data processed by specialized software. OC tests were carried out by adding to the reactor an appropriate quantity of SBBGR biomass, diluted with tap water to give a VSS concentration around 1000 mg/L. Allythiourea (ATU) was added to the biomass in order to inhibit ammonia oxidation. The first step of the procedure was the pre-aeration of the biomass suspension, for at least 4 h, in order to remove the residual COD in the sludge. After that, the sample was oxygenated by pure oxygen until a 20 (mg of O2)/L dissolved oxygen (DO) concentration was reached. The oxygen supply was then turned off, and from the DO profile, the oxygen consumption rate due to endogenous respiration was measured. At this point, a fixed volume of filtered municipal wastewater was added to get the same S/X SBBGR ratio, and the new DO profile was recorded till the endogenous respiration phase was obtained again. Pure oxygen was provided instead of air, in order to ensure more effective oxygenation and easily reach an oxygen concentration high enough to avoid reoxygenation during the test. At the end of the test, a liquid sample was taken, filtered,
Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6663 Table 2. Average Compositions of SBBGR Influents (Municipal Wastewater and Landfill Leachate) during the Investigation
a
parameter
municipal wastewater
COD (mg/L) CODa (mg/L) TKN (mg/L) NH4-N (mg/L) NO3-N (mg/L) pH PO4-P (mg/L) TSS (mg/L) VSS (mg/L)
400 200 56.2 37.5 absent 7.6 2 160 133
pretreated landfill leachate 24 400 23 026 160 128 absent 8.4 14 1 540 1 300
Filtered sample.
and analyzed in order to calculate the amount of COD removed during the test. A filtered municipal wastewater was used instead of an unfiltered sample in order to avoid the absorption of suspended material on the biomass surface. The oxygen consumption due to substrate consumption was obtained by measuring the difference between the oxygen consumption rates with and without wastewater. 2.3. Wastewater Composition. During period A, the SBBGR treated primary effluent coming from a municipal wastewater treatment plant (200 000 p.e.), located in Bari (southern Italy), while in period B, it was fed with a pretreated leachate coming from a mature municipal landfill located in Apulia, a region of southern Italy. During the investigated period, the average compositions of the two treated wastewaters were as reported in Table 2.
Figure 2. COD and TSS concentrations in the effluent and their relative removal efficiencies as functions of applied organic load during period A.
3. Results and Discussion 3.1. Municipal Wastewater Treatment. The results obtained during period A (treatment of municipal wastewater) are reported in Figure 2. In particular, this figure reports COD and total suspended solids concentrations recorded in the effluent as well as their removal efficiencies all as functions of the organic load applied. Looking at Figure 2, it is possible to note that the effluent COD concentration was always lower than 50 mg/L with removal efficiencies around 90%, even when the maximum organic load (i.e., 7.0 (kg of COD)/m3‚d) was applied. Considering the high COD removal efficiencies obtained at this organic loading value (7 (kg of COD)/m3‚d), it might have been possible to increase the organic load further. However, this was not attempted, since nitrogen removal was greatly affected at this organic loading value, as reported below. Nevertheless, this result is of great interest when compared either to those obtained by activated sludge systems (commonly characterized by volumetric conversion capacities
