Sludge Loading Rate Research Articles

Sludge treatment and drying reed bed systems

Sludge Reed beds have been used for dewatering (draining and evapotranspiration) and mineralisation of sludge in Denmark since 1988 when the first sludge processing system was introduced. Sludge from wastewater treatment plants (2500–125 000 PE) is treated in sludge reed bed systems with 1–18 basins with loading rates of 25–2200 tonnes dry solids/year for 10 years. In 2002, approximately 95 systems were in operation. Dimensioning and design of reed bed systems depends on the sludge production rate, sludge type, quality and regional climate.The maximum sludge loading rate is approximately 50–60kg DS m−2 year−1. Loading cycles are related to the sludge type and the age of the sludge reed systems. The sludge residue will, after approximately 10 years of operation, reach an approximate height of 1.2–1.5 metres with dry solids content of 30%. The concentration of LAS and NPE (total) in the sludge residue was reduced by a total of approximately 98% and 93%, respectively. Experience has shown that the quality of the final product with respect to heavy metals, hazardous organic compounds and pathogen removal after 10 years of treatment make it possible to recycle the biosolids to agriculture as an Enhanced treated product.

Evaluation by respirometry of the loading capacity of a high rate vermicompost bed for treating sewage sludge

This study examines high rate vermicomposting of sewage sludge using high stocking densities of earthworms. To examine the loading capacity, two vermicompost beds were established in identical 0.84 m diameter reactors, one loaded at an average rate of 10 kg-wet-sludge-mixture m −2 day −1 (0.5 kg-carbon m −2 day −1), the other loaded at 20 kg-wet-sludge-mixture m −2 day −1 (1 kg-carbon m −2 day −1). The sludge mixture was from a commercial vermicomposting company (Vermitech ®) and contained 80–90% sludge and 10–20% green waste and clay. The beds were operated in fed-batch mode for 38 days, and then monitored for a further 12 days without any further sludge loading. Earthworms ( Eisenia fetida) were added once or twice weekly over the 38 days loading period to gradually decrease the ratio of sludge loading rate to total earthworm biomass. By adding earthworm incrementally, the feeding rate ranged from 3.9 to 1.25 kg-wet-sludge kg-earthworm −1 day −1 for the full load experiment and 2–0.62 kg-wet-sludge kg-earthworm −1 day −1 for the half load experiment. The extent of degradation was estimated by fitting a 1st order model to the CO 2 production rate from the beds. Based on the 1st order model, 53 ± 20% (95% CI) and 68 ± 4% of the organic carbon was converted to CO 2–C in the full load and half load experiments respectively. The CO 2 production rate in the half load experiment became stable and repeatable when the total earthworm biomass reached 5.4 kg, corresponding to a feed rate of 1.04 kg-wet-sludge-mixture kg-earthworm −1 day −1. In contrast, the rate of CO 2 production was still climbing and traces of methane were evident in the full load experiment at the end of the 38 day loading period. The experiments indicate that high rate vermicomposting beds are sustainable providing the feeding rate does not exceed approximately 1 kg-wet-sludge kg-earthworm −1 day −1.

Membrane fouling - a review on the role of EPS

Extracellular polymeric substances (EPS) in either bound or soluble/colloidal form are currently considered as the major cause of membrane fouling in membrane bioreactors. Several factors like the type of wastewater, sludge loading rate, sludge age, MLSS concentration, mechanical stress, and microbial growth phase are known to influence the concentration of EPS in one way or another [1–4]. Also, unsteady states like intermittent feeding, irregular sludge wastage or shifts in the oxygen supply [5–7] have been identified as additional factors leading to an increase in EPS formation or to a change in its fouling propensity (Fig. 1). Despite the large number of technical publications on membrane fouling and EPS, many questions remain unanswered to date. A number of attempts have been made to correlate flux with MLSS concentration, floc size, sludge rheology [1], and soluble EPS concentration [8], but due to the complex nature of the biological system and the difference in experimental and analytical methods applied these are inconsistent and often even contradictory. This paper reviews results from the authors’ own and other labs and thereby aims at establishing a general evaluation of the role of EPS for membrane fouling.

Psychrophilic and mesophilic anaerobic digestion of brewery effluent: A comparative study

Two expanded granular sludge bed-anaerobic filter (EGSB-AF) bioreactors (3.38 l active volume) were used to directly compare psychrophilic (15 °C), anaerobic digestion (PAD) to mesophilic (37 °C) anaerobic digestion (MAD) for the treatment of a brewery wastewater (chemical oxygen demand (COD) concentration of 3136±891 mg l −1). Bioreactor performance was evaluated by COD removal efficiency and biogas yields at a range of hydraulic and organic loading rates. Specific methanogenic activity (SMA) assays were also employed to investigate the activity of the biomass in the bioreactors. No significant difference in the COD removal efficiencies (which ranged from 85–93%) were recorded between PAD and MAD during the 194-d trial at maximum organic and hydraulic loading rates of 4.47 kg m −3 day −1 and 1.33 m 3 m −3 day −1, respectively. In addition, the methane content (%) of the biogas was very similar. The volumetric biogas yield from the PAD bioreactor was approximately 50% of that from the MAD bioreactor at an organic loading rate of 4.47 kg COD m −3 day −3 and an applied liquid up-flow velocity ( V up) of 2.5 m h −1. Increasing the V up in the PAD bioreactor to 5 m h −1 resulted in a volumetric biogas production rate of approximately 4.1 l d −1 and a methane yield of 0.28 l CH 4 g −1 COD d −1, which were very similar to the MAD bioreactor. Significant and negligible biomass washout was observed in the mesophilic and psychrophilic systems, respectively, thus increasing the sludge loading rate applied to the former and underlining the robustness of the latter, which appeared underloaded. A psychrotolerant mesophilic, but not truly psychrophilic, biomass developed in the PAD bioreactor biomass, with comparable maximum SMA values to the MAD bioreactor biomass. PAD, therefore, was shown to be favourably comparable to MAD for brewery wastewater treatment and biogas generation.

Influence of unsteady membrane bioreactor operation on EPS formation and filtration resistance

Extracellular polymeric substances (EPS) are considered as the major cause of membrane fouling in membrane bioreactors. Recent studies have revealed a linear relationship between fouling rate (increase of filtration resistance over time) and polysaccharide (PS) concentration [1]. Several factors like the type of wastewater, sludge loading rate, sludge age, MLSS concentration, and mechanical stress are known to influence the concentration of dissolved EPS. Also, unsteady states like intermittent feeding or shifts in the oxygen supply have been identified as additional factors leading to an increase in EPS formation or to a change in its fouling propensity. However, no systematic investigation to quantify such influences has been undertaken so far. This study is aimed at determining the influence of discontinuous excess sludge withdrawal often applied in small decentralised plants and resulting unsteady loading rates on EPS formation and filtration resistance. The general trend of increased fouling rate at higher PS concentrations is confirmed but data show a larger scatter which could be due to a change in the PS structure and hence their fouling potential. The levels of dissolved oxygen and nitrate which also fluctuate more strongly in MBRs with irregular sludge wastage appear to have a profound impact on EPS uptake rate and thereby on EPS concentration.

Advanced treatment of textile wastewater towards reuse using a membrane bioreactor

The aim of this work was to quantify the performance of a membrane bioreactor (MBR) for the treatment of textile wastewater, and to investigate its capability to achieve a water quality meeting reuse criteria. A laboratory-scale MBR unit was fed with textile wastewater originating from a polyester finishing mill. Removal capacity was examined at VLRs ranging between 0.35 and 3.6 g/(l day). In addition, the effect of nutrient addition was studied. COD removal was found to vary between 60 and 95% with reduced COD levels at lower VLRs tested. Nutrient addition slightly enhanced effluent quality. Sludge yield obtained were between 0.01 and 0.1 gMLSS/gCOD removed. At similar sludge loading rates specific sludge production rates were approximately 50% higher when nutrients were added. A distinct relationship between sludge growth and colour removal could be observed. Above an sludge growth rate of 0.3 g/(l day), colour removal was above 87% for all wavelengths examined. If reuse of MBR treated wastewater is intended, additional polishing steps must be considered as MBR effluents did not reach the required quality for water reuse. In order to upgrade MBR effluent, nanofiltration as a post-treatment was suggested.

SLUDGE TREATMENT AND DRYING REED BED SYSTEMS IN DENMARK

ABSTRACTSludge Reed beds have been used for dewatering (draining and evapotranspiration) and mineralisation of sludge in Denmark since 1988 when the first sludge processing system was introduced. Sludge from wastewater treatment plants (2,500‐125,000 pe) is treated in sludge reed bed systems with 1–18 basins with loading rates of 25–2,200 tonnes dry solids/year for ten years. In 2002, approximately 95 systems were in operation. Dimensioning and design of reed bed systems depends on the sludge production rate, sludge type, quality and regional climate.The maximum sludge loading rate is approximately 50–60 kg DS/m2/year. Loading cycles are related to the sludge type and the age of the sludge reed systems. The sludge residue will, after approximately ten years of operation, reach an approximate height of 1.2–1.5 metres with dry solids content of 30–40%. Experience has shown that the quality of the final product with respect to heavy metals, hazardous organic compounds and pathogen removal after ten years of treatment make it possible to recycle the biosolids to agriculture as an enhanced treated product.

Removal of organic pollutants and analysis of MLSS–COD removal relationship at different HRTs in a submerged membrane bioreactor

Abstract In order to investigate the influence of hydraulic retention time (HRT) on organic pollutant removal in a submerged membrane bioreactor (SMBR), a laboratory-scale experiment was conducted using domestic sewage as influent. The dissolved oxygen (DO) concentration was controlled at 2.0– 3.0 mg L - 1 during the experimental period. The experiments demonstrated that when HRT was 3, 2 and 1 h, the reduction of chemical oxygen demand (COD) was 89.3–97.2, 88.5–97.3 and 80–91.1%, and the effluent COD was 38.9–11.2, 41.6–10.8 and 63.4– 35.8 mg L - 1 , respectively. It is suggested that an HRT of 1 h could meet the normal standard of discharged domestic sewage, and an HRT of 2 h could meet that of water reclamation. In addition, we use mathematical software MATLAB to analyse the relation of mixed liquor suspended solids (MLSS) and COD removal. The results showed that the optimum MLSS concentration should be maintained at around 6000 mg L - 1 in the SMBR. The results also showed that the COD removal was related to HRT ( τ ) , influent concentration ( S 0 ) and sludge loading rate for COD removal ( N S ). Moreover, the high COD removal could be achieved through adjusting τ , S 0 and N S .

On-site treatment of a motorway service area wastewater using a package sequencing batch reactor (SBR)

A sequencing batch reactor (SBR) treating the effluent of a motorway service station in the south of England situated on a major tourist route was investigated. Wastewater from the kitchens, toilets and washrooms facilities was collected from the areas on each side of the motorway for treatment on-site. The SBR was designed for a population equivalent (p.e.) of 500, assuming an average flow of 100 m3/d, influent biochemical oxygen demand (BOD) of 300 mg/l, and influent suspended solids (SS) of 300 mg/l. Influent monitoring over 8 weeks revealed that the average flow was only 65 m3/d and the average influent BOD and SS were 480 mg/l and 473 mg/l respectively. This corresponded to a high sludge loading rate (F:M) of 0.42 d(-1) which accounted for poor performance. Therefore the cycle times were extended from 6 h to 7 h and effluent BOD improved from 79 to 27 mg/l.

Characteristics of sludge developed under different loading conditions during UASB reactor start-up and granulation

Sludge characteristics available inside the reactor are of vital importance to maximize advantages of UASB reactor. The organic loading rate and sludge loading rate applied during start-up are among the important parameters to govern the sludge characteristics. Effects of these loading rates on the characteristics of the sludge developed are evaluated in six laboratory scale UASB reactors. The sludge characteristics considered are VSS/SS ratio of the sludge, sludge volume index, specific gravity, settling velocity and metal contents of the sludge developed under different loading rates. The experimental results indicate that, for developing good characteristics sludge, during primary start-up from flocculent inoculum sludge, organic loading rate and sludge loading rate should be in the range of 2.0-4.5 kg COD/m3 d and 0.1-0.25 kg COD/kg VSS d, respectively (chemical oxygen demand, COD). Proper sludge granulation and higher COD removal efficiency will be achieved by these loading rates.

EFFECT OF SLUDGE ON GROWTH AND UPTAKE OF HEAVY METALS BY COMMON BERMUDAGRASS

Sixty soil columns (12 combinations, each having five replicates) on which common Bermudagrass grown were prepared to investigate the feasibility and practicability of sewage sludge as a soil amendment material with focus on the effect of sludge on grass growth and heavy metal (Cd, Cr, Cu, Ni, Pb and Zn) uptake under four sludge loading rates and three lime loading rates. It was found that sludge loading rate of 50 g kg−1 CDG and lime loading rate of 4 g kg−1 CDG gave the greatest above-ground biomass and the lowest concentration of heavy metals in grass tissue under experimental conditions. The extension of sludge use to slope bio-engineering will be discussed which also help to relieve the pressure of sludge disposal in landfills.

Minimization of sludge production and stable operational condition of a submerged membrane activated sludge process

Modification of conventional activated sludge process by adding microfiltration type hollow fiber membrane module offers various advantages. The membrane module is immersed directly in the bioreactor to separate the treated water from the sludge and the modified process is called submerged membrane activated sludge process (SMASP). Bench scale SMASP units were operated at constant flux to study the possibility of minimizing the excess sludge production and stable operation at different SRT and BOD loading. The long SRT developed high stabilized MLSS concentration in the bioreactor and decreased the sludge loading rate [kg-BOD/kg-MLSS-d]. In SMASP, very low sludge yield coefficients (0.04-0.09 kg-MLSS/kg-BOD) were obtained at long SRT (500 d) and small BOD loading (< or =0.5 kg/(m3 x d)) as compared to conventional activated sludge process and these coefficients showed considerable minimization in excess activated sludge production. At the same operation conditions, suction pressure was also maintained low (<25 kPa), and then SMASP was operated stably for long time (approximately 200 d).

Sulfidogenic volatile fatty acid degradation in a baffled reactor

The effect of staging the sludge bed on volatile fatty acid degradation by sulfidogenic reactors was evaluated in a baffled reactor. In a 5.4 l baffled reactor, containing three equal compartments, a volatile fatty acid (VFA) mixture (acetate:propionate:butyrate ratio 1:2:2 on COD basis; pH 8) was treated under mesophilic (30 degrees C) and sulfidogenic (COD:SO4(2-) ratio: 0.5) conditions for 38 days. At a specific sludge loading rate of 0.50 g COD.gVSS(-1) x d(-1), a COD and sulfate removal of 85% and 30%, respectively, was obtained. In the baffled reactor, staging of the sulfidogenic VFA degradation occurred. Propionate and butyrate were mainly degraded in the first compartment. Their degradation was incomplete, resulting in elevated acetate concentrations in compartment I. In the second and third compartment of the baffled reactor, a net degradation of acetate took place. Acetate was the sole substrate present in compartment III and residual acetate concentrations of about 200 mg/l were present in the effluent at a specific sludge loading of 0.50 g COD x gVSS(-1) x d(-1). Sludges with different maximum specific VFA and acetate degrading activities developed in the first and second compartment. These maximal specific activities were almost equal for sludge present in compartment II and III.

Coarse media filtration - an alternative to settling in wastewater treatment

In this paper coarse media filtration has been analysed as an alternative to the traditional settling in primary, secondary and tertiary treatment of wastewater. Various filter media configurations were evaluated for enhanced primary filtration. It was found that a dual media configuration based on Kaldnes biofilm media (1 and K2modified) were most suitable when taking both separation efficiency as well as filter run time into consideration. SS removal efficiencies of around 75% were achieved in the dual Kaldnes primary filter at around 20 m/h without any chemical addition and around 85% at low dosage (1-2 mg/l) of a high MW cationic polymer FO4440SH. In the latter case COD was removed by around 70%. Further experiments were carried out on a multi-media Kaldnes-Filtralite-Sand (KFS) filter for enhanced primary treatment as well as for secondary filtration directly downstream of a high-rate moving bed biofilm reactor, resulting in an extremely compact secondary treatment process. The secondary KFS-filter gave SS-removal efficiencies around 90% (effluent SS < 15 mg/l) and filter run times of around 24 hrs at filtration rates of 10 m/h (sludge loading rates of around 1 kg/m2h) when a small dose (2 mg/l) of polymer was used. It is also demonstrated that the primary filter may also be utilised as a pre-denitrification reactor. A denitrification rate of 1.5 kg NO3-N(equiv.)/m3d was achieved when the filter was operated at a filtration rate of 5 m/h.

The effect of organic loading rate on the performance of UASB reactor treating slaughterhouse effluent

Organic loading rate (OLR) is an important parameter significantly affecting microbial ecology and characteristics of UASB systems. In this study, UASB performance was evaluated in a 1000 1 reactor receiving feed from a traditional medium-size slaughterhouse. The initial seed for granules formed earlier was from a mesophilic municipal anaerobic digester sludge with a VSS content of 29 g l −1. The temperature of influent was adjusted by an inline thermostat around 33 °C. The reactor was started with an OLR of 5 kg SCOD m −3 d −1 with gradual increase to 10 kg SCOD m −3 d −1 over a 2-week period. Examination of VSS data showed that on the average 89.3±11.3% of bioparticle mass was present at the lower 30% of the reactor height. Under steady state conditions, experiments were conducted at OLRs of between 13 and 39 kg SCOD m −3 d −1 and hydraulic retention times (HRT) of 2–7 h. Removal efficiencies in the range of 75–90% were achieved at feed SCOD concentrations of 3000–4500 mg l −1. A reduction in removal efficiency to as low as 67% could have been related to a combined effect of high OLR and low HRT. Up to 300 l of methane were produced per kilogram of SCOD removed at OLR values of less than 30 kg COD m −3 d −1 but methane production rate seemed to decline to below 200 at higher OLR values. No sign of cell washout was observed at high OLRs and sludge loading rates (SLR) of up to 2.7 kg SCOD kg −1 VSS d −1. Elimination capacity of the reactor consistently increased from 9 to 25 kg SCOD m −3 d −1 corresponding to 1–2 kg SCOD kg −1 VSS d −1. Solids retention time (SRT) calculations for the reactor indicated a range of 3.3 days at high upflow velocity of 1 m h −1 to 60.3 days at low upflow velocity of 0.33 m h −1 s during different phases of the study.

Environmental risks of applying sewage sludge compost to vineyards: carbon, heavy metals, nitrogen, and phosphorus accumulation.

Biosolids are applied to vineyards to supply organic matter. However, there is concern that this practice can increase the concentration of macronutrients and heavy metals in the soil, some of which can leach. We evaluated the environmental hazard of sewage sludge compost applied in March 1999 at 10, 30, and 90 Mg ha-1 fresh weight in a vineyard in southeastern France. Soil organic matter increased in all plots by 3 g kg-1 18 mo after the amendment. Neither total nor available heavy metal concentrations increased in the soil. Mineral nitrogen (N) in the topsoil of amended plots of 10, 30, and 90 Mg ha-1 increased by 5, 14, and 26 kg (NO3(-)-N + NH4(+)-N) ha-1, respectively, the first summer and by 2, 5, and 10 kg (NO3(-)-N + NH4(+)-N) ha-1, respectively, the second summer compared with controls. At the recommended rate, risks of N leaching is very low, but phosphorus (P) appeared to be the limiting factor. Phosphorus significantly increased only in plots amended with the highest rate in the topsoil and subsoil. At lower rates, although no significant differences were observed, P added was greater than the quantities absorbed by vines. In the long run, P will accumulate in the soil and may reach concentrations that will pose a risk to surface waters and ground water. Therefore, although the current recommended rate (10 Mg ha-1) increased soil organic matter without the risk of N leaching, total sewage sludge loading rates on vineyards should be based on P concentrations.

The use of upflow anaerobic sludge blanket reactors in the treatment of poultry slaughterhouse wastewater

This work studied the performance of the dissolved air flotation (DAF) system and the start-up and the operation of two 450 m3 UASB reactors in a poultry slaughterhouse in Sorocaba, Brazil. The DAF presented reduction efficiency of grease and fats, suspended solids and COD 50% higher. The reactors were seeded with non-adapted sludge. The average COD of the reactor influent was 2,695 mg/L; and the initial organic loading rate (OLR) and the initial sludge loading rate at the start-up were 0.51 kg COD/m3.day and 0.04 kg COD/kg VTS.day, respectively. The start-up period was 144 days. During this time the reactor flow rate and OLR were gradually increased. At the reactor start-up, the maximum OLR value was 2.1 kg COD/m3.day, the COD reduction was higher than 80%, and the concentration of volatile fatty acids (VFA) was below 100 mg/L. The COD reductions, considering the reactor effluent raw COD and soluble COD were similar throughout the period studied in both reactors. The reactor effluent raw COD was approximately 10% higher than the soluble COD until the 225th day of operation. From the 225th day of operation this value increased 20%-30% due to the sludge washout. The effluent soluble COD reduction, the effluent VFA concentration and the operational stability attested the good performance of UASB reactors in poultry slaughterhouse wastewater treatment.

Anaerobic–aerobic biotreatment of simulated textile effluent containing varied ratios of starch and azo dye

Combined anaerobic–aerobic treatment was used to treat a simulated textile industry wastewater (overall HRT 1.8 days). The azo dye (PROCION Red H-E7B) and starch concentrations were varied in a series of 1-week experiments to determine the effect of starch:dye ratio on COD, BOD and colour removal. The treatment efficiency of the system at 1.9 g l −1 starch and 0.15 g l −1 dye remained constant over 130 days despite seven intervening 1-week periods of operation at other starch:dye ratios. Most colour removal occurred in the UASB reactor (1 d HRT), and the BOD:COD of the UASB reactor effluent increased by up to 47%. The maximum overall COD removal was 88% and the BOD removal was up to 99%. A maximum of 77% colour removal overall was achieved with starch and dye concentrations of 3.8 and 0.15 g l −1, giving a final true colour of 0.21 TCU. At both 0.15 and 0.75 g l −1 dye a starch concentration of 3.8 g l −1 rather than 1.9 g l −1 gave a significant improvement in colour removal. However, at 3.8 g l −1 starch volatile fatty acid levels in the UASB reactor rose, while at 2.9 g l −1 starch they did not. It is recommended that if colour removal efficiency decreases, carbohydrate should be added to the anaerobic reactor at a maximum sludge loading rate between 0.11 and 0.15 kg COD kg −1 TVS d −1.

Pulp Sludge as a Component in Manufactured Topsoil

AbstractThe primary objective of this study was to evaluate the use of uncomposted, de‐watered pulp sludge as the organic matter component in a manufactured topsoil. Seven manufactured topsoils, containing 5.1, 8.8, 9.6, 10.9, or 13.8% pulp sludge and 0, 8.4, or 20.7% flume grit on a dry weight basis, were applied to an abandoned gravel pit. Manufactured topsoils and a control topsoil were evaluated for (i) impacts on soil and soil solution chemistry and (ii) effectiveness as a growing medium for a grass conservation mix and hybrid poplars (Populus spp.). Significant N mineralization was evident in all of the manufactured topsoils within the first field season. Soil cation exchange capacity (CEC), pH and P availability were positively correlated to pulp sludge loading rate. In soil solution, the highest concentrations of solutes were detected within 2 mo of topsoil placement and were dominated by NO3‐N and Ca. Cumulative grass yields from the 15 mo following topsoil placement were greater than those in the control topsoil and ranged from 3.9 to 7.3 Mg ha−1 in the manufactured topsoils. Tree height, diameter growth, and foliar nutrient concentrations responded positively to the manufactured topsoils. Results from this study indicate that topsoils manufactured with pulp sludge as the organic matter component can be an environmentally sound alternative to natural topsoil for the reclamation of sites on which existing conditions necessitate importing topsoil for revegetation.

Effect of nitrate on acetate degradation in a sulfidogenic staged reactor

The optimisation of acetate removal in sulfidogenic reactors by adding nitrate to the last compartment of a baffled reactor was evaluated. In a 5.4 l baffled reactor, containing three equal compartments, a VFA mixture (acetate:propionate:butyrate ratio 1:2:2 on COD basis; pH 8) was treated under mesophilic (30°C) and sulfidogenic (COD:SO 2− 4 ratio: 0.5) conditions. At a specific sludge loading rate of 1.1 g COD gVSS −1 d −1, a COD and sulfate removal of 80 and 35%, respectively, was obtained. In the baffled reactor, staging of the sulfidogenic VFA degradation occurred. Propionate and butyrate were mainly degraded in the first compartment. Their degradation was incomplete, resulting in elevated acetate concentrations in Compartment I. In the second and third compartment of the baffled reactor, a net degradation of acetate took place. Acetate was the sole substrate present in Compartment III and residual acetate concentrations of about 600 mg/l were present in the effluent at the highest specific sludge loading applied. In the three compartments, development of sludges with different maximum specific acetate degrading activities was observed. The maximal specific activity with acetate was three times higher in sludge growing in Compartment III as compared to sludge growing in Compartment I. Batch experiments showed that the addition of about 500 mg/l nitrate (COD/nitrate ratio of 2.5) increased the specific acetate degradation rate by 300%. Nitrate was denitrified (pH optimum of 7.0–8.0) with a temporary accumulation of the intermediate nitrite. When 2.3 g/l nitrate (COD/nitrate ratio of 0.5) was dosed in compartment III of the baffled reactor, the effluent acetate concentration could be decreased from 600 to 100 mg/l.