Biological nutrient removal by a sequencing batch reactor (SBR) using an internal organic carbon source in digested piggery wastewater

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1 Bioresource Technology 96 (25) 7 1 Biological nutrient removal by a sequencing batch reactor (SBR) using an internal organic carbon source in digested piggery wastewater D. Obaja, S. Mace, J. Mata-Alvarez * Departament d Enginyeria Quımica i Metallurgia, Universitat de Barcelona, Martı i Franques, no. 1, pta. 6, 828 Barcelona, Spain Received 5 September 23; accepted 5 March 2 Available online 16 April 2 Abstract Experiments in a lab-scale SBR were conducted to demonstrate the feasibility of using an internal carbon source (non-digested pig manure) for biological nitrogen and phosphorus removal in digested piggery wastewater. The internal C-source used for denitrification had similar effects to acetate. 99.8% of nitrogen and 97.8% of phosphate were removed in the SBR, from an initial content in the feed of 9 mg/l ammonia and 9 mg/l phosphate. Ó 2 Elsevier Ltd. All rights reserved. Keywords: Piggery wastewater treatment; Internal carbon source; Sequencing batch reactor; Nutrient removal; Nitrification; Denitrification; Biological phosphorus removal 1. Introduction Biological denitrification is a reliable method for nitrogen removal from wastewater (Abufayed and Schroeder, 1986). Heterotrophic bacteria use the available carbon source. Since nitrified liquor is usually deficient in organic carbon and the low carbon source level limits the biological denitrification process, sufficient organic carbon sources must be provided for proper denitrification. In addition, for proper biological phosphorus removal, an easily biodegradable carbon source is needed at the P release stage. Effluents from pig manure anaerobic digesters still contain a large amount of COD and a large nutrient load (ammonia and phosphate). Though a small fraction of the COD consists of volatile fatty acids and could be used for denitrification, easily biodegradable COD is clearly needed, given the amounts of nitrogen and phosphorus. Synthetic chemicals, such as methanol or acetic acid, are expensive, but are quite effective. However, the use of non-digested pig manure as a source of easily biodegradable carbon is an alternative and cheaper method. The use of wastewater * Corresponding author. Tel.: ; fax: address: jmata@ub.edu (J. Mata-Alvarez). as an electron donor for denitrification in sequencing batch reactors (SBRs) was suggested by Pallis and Irvine (1985) and implemented for piggery wastewaters by Bortone et al. (1992). This approach has aroused greater interest recently, especially the use of domestic wastewater with the organic fraction of municipal solid waste (OFMSW) as carbon source. For instance, Beccari et al. (1998) studied the potential of readily biodegradable COD obtained from acidogenic fermentation of the OFMSW as electron donor for denitrification. Experiments at an SBR pilot plant used domestic wastewater from the wastewater treatment plant at Fusina (Venice). The results showed a remarkable improvement both in denitrification rate and in flexibility of the response to influent load peaks. The main advantage of this approach is economic, as no expensive external carbon source is required. Other studies have followed this idea of using residual effluents as substitutes for chemical carbonaceous sources (e.g. Monteith et al., 198; Skrinde and Bhagat, 1982; Bernet et al., 1996; Lee et al., 1997; Ra et al., 2; Graja and Wilderer, 21; Cervantes et al., 21). This study aimed to establish an approach to removing nutrients from digested pig manure in an SBR with internal easily degradable organic matter and to explore the feasibility of this more cost-efficient removal /$ - see front matter Ó 2 Elsevier Ltd. All rights reserved. doi:1.116/j.biortech.2.3.2

2 8 D. Obaja et al. / Bioresource Technology 96 (25) 7 1 system. SBRs provide a simple and even economical way of treating piggery wastewaters. SBR systems offer substantial benefits over alternative conventional flow systems, because of their flexibility and capacity to meet various treatment objectives (Norcross, 1992; Ketchum, 1997). In the experiments, non-digested pig manure was added at the beginning of the denitrification stage, together with acetic acid in different ratios. Reactor performance was then studied under three operating conditions (P1, P2 and P3), with different proportions of external (acetic acid) and internal (volatile fatty acids of pig manure) carbon. The appropriate C/N ratios were established with care, so as to achieve complete denitrification without any carbon source excess. Thus, both waste of time and oxygen consumption were avoided. Table 1 Characterisation of the digested and non-digested piggery wastewater supernatant after centrifugation Parameters Digested piggery wastewater Non-digested piggery wastewater ph TS (g/l) VS (g/l) TSS (g/l) VSS (g/l) VFA (mg/l) BOD 5 (mg/l) COD (mg/l) Alkal 7:7 (mg/l) Alkal 3:53 (mg/l) Total N (mg/l) NH þ -N (mg/l) NO 3 -N (mg/l) 6 -N (mg/l) 6 NO 2 -P (mg/l) No. of analyses 2. Methods 2.1. Substrate and readily biodegradable C-source Speed Regulator Timers T1 T2 T3 T ph meter Raw and digested pig manure was obtained from a piggery at Caldes de Montbui, 3 km from Barcelona. The latter had been treated in an industrial anaerobic digester on the farm. The effluent from the digester was centrifuged (Beckman model J2-21) at rpm for 15 min at 5 C, to remove most of the suspended solids. The supernatant was used as substrate for the experiments. The non-digested pig manure was used after centrifugation as an easily biodegradable carbon source. Before being fed into the reactor, the supernatant was diluted with tap water to the desired concentration (see Fig. 1 for feed preparation). Table 1 shows the characteristics of the supernatant and of the non-digested pig manure. Influent Effluent Air Stirrer ph HCl 2.2. Experimental device PT1 The SBR, a cylindrical tank with a volume of 3 l, was made of Pyrex glass and fitted with mixing and air sparging systems (Fig. 2). It was complemented by two peristaltic pumps (Cole Parmer Instruments, Model DIFFUSER Fig. 2. Diagram of the SBR. REACTOR SBR Feed reservoir + NH -N concentration = 9 mg/l Piggery wastewater reservoir NH + -N concentration = 16 mg/l (Table 1) To be diluted with SBR effluent (here, with tap water) Fig. 1. Feed preparation: after feed is prepared, it is introduced in the reactor. As there are 2 l remaining from the previous cycle, the initial concentration of NH þ -N is 3 mg/l. number , Chicago), one for feeding, the other for drawing off effluent and excess sludge. The SBR operation cycles were controlled by programmable timers. A data acquisition system (model CRISON phrocon 18, Barcelona) was used for the continuous recording of the mixed liquor temperature and of ph. Optimum ph was set at 8.1. Higher ph was corrected by mechanical addition of hydrochloric acid HCl (.5 N).

3 D. Obaja et al. / Bioresource Technology 96 (25) Analytical methods Chemical oxygen demand (COD), biochemical oxygen demand (BOD 5 ), alkalinity, total nitrogen (TN) and solids (total suspended solids (TSS), volatile suspended solids (VSS), total solids (TS) and volatile solids (VS)) were all analysed following standard methods (APHA, 1992). Nitrogen compounds, nitrates, nitrites and phosphates were analysed with an ionic chromatograph (KONIK model KNK-5-A Series, column Waters IC. Pak) under the following conditions: automatic injector (Kontron model HPLC autosampler 65), 8 ll volume sample, conductivity detector (Wescan) for concentrations between 2 and 5 mg/l, and UV detector (Kontron HPLC 332) for low concentrations (.1 2 mg/l). Ammonium was determined by an ammonia-specific electrode (Crison, model ph 22, Barcelona). Volatile fatty acids (VFA) were analysed by gas chromatography (HP 589 Series II, flame ionisation detector) under the following conditions: column from 12 to 17 C, injector at 28 C, detector at 3 C and volume sample 1 ll. Helium was employed as carrier gas. Samples were centrifuged at 1, rpm for 1 min and filtrated through.5 lm to remove suspended solids prior to being fed to the chromatographic columns. 3. Results and discussion 3.1. Optimisation of the cycle length In a preliminary study using acetic acid as carbon source (Obaja et al., 23), a set of optimal conditions for removing nutrients from digested pig manure was established. This consisted of three cycles of 8 h per day. The maximum initial NH þ -N concentration that could be treated in the reactor was 5 mg/l, with a hydraulic retention time (HRT) set at one day. Thus, the total N removed was 15 mg NH þ -N/l day. However, later optimisation experiments (Obaja, 22) improved removal efficiency. In these experiments, several cycle lengths,, 8, 12 and 2 h, were tested. In all of them, a constant quantity of effluent was fed per day (1/ 3 of reactor volume). As the number of cycles per day was increased and the quantity of substrate fed per day remained constant, the total amount of substrate fed was increased and the HRT was decreased. The results obtained in these optimisation experiments are shown in Table 2 and Fig. 3. As can be seen, the best yields in terms of nitrification rate were obtained in -h cycles. With this strategy, the maximum initial NH þ -N concentration that could be treated in the reactor was found to be 3 mg/l per cycle with an HRT set at.5 days. Thus, the total N removed in 6 cycles/day was 18 mg NH þ -N/l day. Therefore, this was the initial strategy chosen to remove the ammonium and organic matter from the influent with an internal carbon source for denitrification Optimisation of the dose of easily degradable carbon Reactor performance was studied under three operating conditions (experiments P1, P2 and P3), of which external (acetic acid) and internal (volatile fatty acids of pig manure) carbon made up, respectively, 75% and 25%, 5% and 5%, and % and 1% (% expressed in terms of mg of acetic acid). All these experiments were Table 2 Influence of the cycle length Cycle s length (h) Initial NH þ -N concentration in reactor (mg/l) Average nitrification rate (mg NH þ -N/l h) NH þ -N removal (%) NO x -N removal (%) -P removal (%) Maximum N removal (mg N-NH þ -N/day l) HRT (days) (mg/l) NH -N NO 3 -N NO 2 -N PO -P Fig. 3. Profiles of the concentrations of the different species of N and P obtained during the cycle of h.

4 1 D. Obaja et al. / Bioresource Technology 96 (25) 7 1 conducted at 25 C (estimated as a minimum for the temperature of the digester effluent after centrifugation in a full-scale plant). Each cycle began with an anaerobic stage of 1 h (the reactor was stirred but not aerated) for phosphorus removal. Between the second and third hour of the cycle there was an aerobic stage, with stirring and aeration, for nitrification. The fourth hour was anoxic for denitrification, adding initially the easily biodegradable carbon source, with stirring but no aeration. The cycle should have finished at this stage. However, as the main effect of adding the internal carbon source was to increase NH þ -N concentration in the reactor, the cycle length had to be extended. In fact, three additional hours were needed (intermittently aerobic and anoxic every 3 min) to remove this nutrient, resulting in an HRT of.87 days. The operating conditions of this new cycle length are shown in Table 3. After 7 h, the sludge was drawn, with stirring and aeration throughout the stage to ensure a homogeneous purge. Thus, SRT (or sludge age) was kept constant and equal to 11 days in all the experiments (Bortone et al., 199). The solids then settled and liquid was drawn (approximately 1/3 of the reactor supernatant in each cycle including the volume of the purge) in order to maintain the HRT level set. Then a new cycle started Period 1 In this period, the internal carbon source was established as 25% from the VFA supplied by the wastewater and 75% as acetic acid. During the aerobic stage, NH þ -N concentration decreased from 3 to 1 mg/l (i.e. 99.7% NH þ -N removal). The average nitrification rate was 19.5 mg/l h, a figure higher than other authors results, which oscillate mainly between 3 and 15 mg/l h (Bortone et al., 1992, Table 3 Operational parameters of the lab-scale SBR used in this study Parameters (in the reactor) Experiment P1 P2 P3 Volume (l) Influent flow rate (l/day) Cycle length (h) Number of cycles per day Influent volume per cycle (l) mg of NH þ -N removed per day and litre of reactor HRT (day) SRT (day) ph <8.1 <8.1 <8.1 Temperature ( C) % of internal C-source a Number of easily biodegradable C-source additions during the cycle a Remaining up to 1 was acetic acid. 199; Andreottola et al., 1997; Lee et al., 1997; Su et al., 1997; Tilche et al., 1999, 21; Edgerton et al., 2; Ra et al., 2). According to Bernet et al. (1996), at the denitrification stage, the C/N ratio needed for a complete removal of NO 3 -N and NO 2 -N is 1.7. Therefore, at the third hour of the cycle (anoxic stage), 113 mg/l of VFA were added (25% coming from non-digested pig manure and 75% acetic acid). Fig. A shows the profiles of NH þ -N, NO 3 -N and NO 2 -N during this cycle. During this phase, which lasted 1 h, nitrate concentration decreased from 257 to mg NO 3 -N/l and all the VFA introduced were consumed. When the 113 mg/l of VFA were introduced into the reactor, 7 mg/l of NH þ - N from the internal carbon source were introduced at the same time. To remove these, another nitrification stage was added, resulting in a nitrate concentration of 6 mg/l. As this stage had to be shorter, only 1 h was programmed for nitrification, as against the 2 h of the previous stage. For complete denitrification, 153 mg/l of VFA were necessary. Following the same strategy for supplying easily degradable carbon, 35 mg/l of NH þ -N were introduced at the same time, which again was removed through a new nitrification stage that produced 31 mg/l of NO 3 -N. By this method, following the cycles consisting of successive phases of nitrification denitrification, all the nutrients were completely removed. Easily degradable C-source was added six times during the entire cycle. Fig. A and B show the complete set of stages (7 h overall) required for a complete removal of nitrogen, using 25% of VFA coming from non-digested pig manure. The first stage of phosphate removal, during the first hour of the cycle, saw an increase in phosphate concentration from 29.2 to 31.9 mg -P/l. During the two hours of the aerobic stage, this concentration decreased from 31.9 to 2.9 mg/l: 91% phosphate removal (see Fig. C). The value of the P/C ratio was Periods 2 and 3 All these results for P1 are summarised in Table and Fig. A C. The results for P2 and P3 are also reported in Table and are shown in Figs. 5A C and 6A C, respectively. As Figs. 6 and Table show, results were very similar and good in the three periods: nutrient removals ranged from 97.8 to In all periods, profiles of ammonia, nitrite and nitrate in the reactor were comparable. Introduction of easily biodegradable carbon source at the beginning of the anoxic phase led to a high rate of denitrification, with all nitrate removed within the first hour of this phase. The almost identical performance of 1% internal carbon feeding source and 25% internal/75% external carbon feeding source suggests that the internal carbon source (non-digested pig manure) is a viable choice for

5 D. Obaja et al. / Bioresource Technology 96 (25) (A) NH -N NO 3 -N NO 2 -N (B) PO -P (C) VFA Fig.. Profiles of the concentrations of the different species of N and P in a single cycle of the SBR operation during period 1. Table Results of the three periods studied and of a previous experiment with acetic acid as carbon source Period 1 Period 2 Period 3 Previous operation without internal C-source Nitrification Influent NH þ -N concentration (mg/l) Effluent NH þ -N concentration (mg/l) Average nitrification rate (mg/l h) % Removal Denitrification Initial NO 3 -N concentration (mg/l) Final NO 3 -N concentration (mg/l) Consumed VFA (mg/l) % of VFA coming from the non-digested pig manure Phosphorus removal -P concentration (mg/l) at the beginning of the anaerobic stage -P concentration (mg/l) at the end of the anaerobic stage -P concentration (mg/l) at the end of the aerobic stage % Removal In all the experiments, values of C/N and P/C were maintained constant and equal to 1.7 and.5 respectively. enhancing SBR performance. The extra nitrogen added by the internal carbon source offers no impediment to either nitrogen or phosphorus removal. Comparison of these results with those obtained in the operation without internal C-source (Table ) shows that results are very similar in terms of removal yields, but are better

6 12 D. Obaja et al. / Bioresource Technology 96 (25) 7 1 (A) NH -N NO 3 -N NO 2 -N (B) PO -P (C) VFA Fig. 5. Profiles of the concentrations of the different species of N and P in a single cycle of the SBR operation during period 2. with an internal carbon source, as the average nitrification rate obtained is around double. Good nutrient removal results are not surprising, since acetate constitutes 5% of products from the fermentation of pig manure (Lee et al., 1997) and is rapidly consumed and degraded by heterotrophic bacteria. However, it has to be stressed that, in all the periods using internal carbon source, three additional hours were needed to remove the added nutrients. In consequence, the length of the cycle and thus the HRT need to be extended considerably Batch experiments In each period studied, the biomass in the activated sludge was characterised by the determination of ammonia utilization rate (AUR), nitrogen utilization rate (NUR) and oxygen utilization rate (OUR). The AUR test can also be used to identify possible inhibitory effects from wastewater. AUR, NUR and OUR increase when the percentage of internal carbon source increases. In fact, when the % of non-digested pig manure is higher, more nutrients are added, which gives higher rates of AUR, NUR and OUR because of the greater activity of the viable biomass (Table 5). Higher AUR, NUR and OUR show that functional groups in activated sludges are sensitive to the specific characteristics of their substrate. Table 5 also compares the average nutrient removal rate, the AUR, the NUR and the OUR of the experiment with an internal carbon source, and the values obtained in a previous experiment (Obaja et al., 23), in which acetic acid was the sole carbon source. Using the same concentration of NH þ -N, AUR, NUR and OUR were higher in experiments with non-digested pig manure used as internal C-source than in experiments with acetic acid. Thus, the type and dose of organic carbon source affect the type of bacteria that develop and the organism growth rate, making the viable biomass more active (Tam et al., 199). 3.. Economic aspects Comparison of experiments with internal and external carbon sources for denitrification makes it clear that the advantage of an internal carbon source is the saving from not having to use an expensive easily biodegradable carbon source. However, there is another factor to take into account, and this has a negative effect on the economics of this approach. As Table 3 shows, in experiments using the internal C-source, 128 mg of NH þ -N per day and litre of

7 D. Obaja et al. / Bioresource Technology 96 (25) (A) (B) (C) NH -N NO 3 -N NO 2 -N PO -P VFA Fig. 6. Profiles of the concentrations of the different species of N and P in a single cycle of the SBR operation during period 3. Table 5 Average of nutrient removal rates, values of AUR, NUR and OUR, and percentage of nutrient removals obtained during the three periods Parameters Units P1 P2 P3 Acetic acid as carbon source (Obaja et al., 23) Ammonium removal rate mg NH þ -N g VSS 1 h Nitrate removal rate mg NO 3 -N g VSS 1 h Phosphate removal rate mg -P g VSS 1 h AUR mg NH þ -N g VSS 1 h NUR mg NO 3 -N g VSS 1 h OUR mg O 2 g VSS 1 min Ammonium removal % Nitrate removal % Phosphate removal % reactor can be removed in 3.3 cycles (7 h long), feeding 1/3 of the reactor volume in each cycle. However, a much better yield in terms of N removal per volume of reactor is obtained with acetic acid used as carbon source. With this latter chemical, 18 mg of NH þ -N per day and litre of reactor can be removed, using six cycles per day ( h long) (see Table 2). This means that, if the internal C-source is used, a larger amount (75% more) is needed to remove the same amount of NH þ -N. In addition, if fresh pig manure is to be used for the substrate needed for denitrification, equipment to prepare this substrate (filter, pump, etc.) should also be provided. In consequence, higher investment is needed, if no raw acetic acid (or methanol) is used for the anoxic step. The two aspects need to be balanced, though the high prices of chemicals should make the period needed to recover the additional investment short. The resource conservation implicit in the use of an internal C-source is also a factor not an economic one, but a question of sustainability that should be taken into account.

8 1 D. Obaja et al. / Bioresource Technology 96 (25) 7 1. Conclusions The sequencing batch reactor is an efficient tool for biological carbon and nutrient removal, capable of achieving effluents with very low nitrogen and phosphorus concentrations from highly concentrated wastewaters. With digested piggery wastewater, nitrogen and phosphorus removal yields are around 1% and 98%, respectively. The time cycle and the duration of each single phase within the time cycle must be designed properly, in order to optimise the removal of nutrients. The choice of hydraulic residence times and cellular retention times will depend on this optimisation. Denitrification can be seen not only as a way of removing nitrogen pollution, but also as an efficient method of removing organic carbon. Complete denitrification was obtained when the C/N ratio was equal to or higher than 1.7. The experiments carried out demonstrated the feasibility of using non-digested pig manure as an easily biodegradable carbon source for denitrification and dephosphatation. The process is as simple as when acetic acid or methanol is used. Indeed, the same excellent nutrient removal is obtained, and the additional carbon and NH þ -N introduced can also be removed. The denitrification rate is affected by the type and dose of organic carbon source used. Higher mean removal rates are obtained with an internal carbon source. This can be explained by the fact that different types of organic carbon sources influence the type of bacteria that develop, organism growth rate, nitrate reduction and the degree of accumulation of intermediate byproducts (Tam et al., 1992, 199). The main advantage of using an internal carbon source is the saving in chemicals. This has a very positive effect on the plant s operating costs. 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