The effect of harvesting on biomass production and nutrient removal in phototrophic biofilm reactors for effluent polishing

An increasing number of wastewater treatment plants require post-treatment to remove residual nitrogen and phosphorus. This study investigated various harvesting regimes that would achieve consistent low effluent concentrations of nitrogen and phosphorus in a phototrophic biofilm reactor. Experiments were performed in a vertical biofilm reactor under continuous artificial lighting and employing artificial wastewater. Under similar conditions, experiments were performed in near-horizontal flow lanes with biofilms of variable thickness. It was possible to maintain low nitrogen and phosphorus concentrations in the effluent of the vertical biofilm reactor by regularly harvesting half of the biofilm. The average areal biomass production rate achieved a 7 g dry weight m^?2 day^?1 for all different harvesting frequencies tested (every 2, 4, or 7 days), corresponding to the different biofilm thicknesses. Apparently, the biomass productivity is similar for a wide range of biofilm thicknesses. The biofilm could not be maintained for more than 2 weeks as, after this period, it spontaneously detached from the carrier material. Contrary to the expectations, the biomass production doubled when the biofilm thickness was increased from 130 μm to 2 mm. This increased production was explained by the lower density and looser structure of the 2 mm biofilm. It was concluded that, concerning biomass production and labor requirement, the optimum harvesting frequency is once per week.     

Characterization of a novel micro-pressure swirl reactor for removal of chemical oxygen demand and total nitrogen from domestic wastewater at low temperature

A novel micro-pressure swirl reactor (MPSR) was designed and applied to treat domestic wastewater at low temperature by acclimating microbial biomass with steadily decreasing temperature from 15 to 3 °C. Chemical oxygen demand (COD) was constantly removed by 85% and maintained below 50 mg L^?1 in the effluent during the process. When the air flow was controlled at 0.2 m³ h^?1, a swirl circulation was formed in the reactor, which created a dissolved oxygen (DO) gradient with a low DO zone in the center and a high DO zone in the periphery for denitrification and nitrification. 81% of total nitrogen was removed by this reactor, in which ammonium was reduced by over 90%. However, denitrification was less effective because of the presence of low levels of oxygen. The progressively decreasing temperature favored acclimation of psychrophilic bacteria in the reactor, which replaced mesophilic bacteria in the process of treatment.     

Kinetic Modelling and Characterization of Microbial Community Present in a Full-Scale UASB Reactor Treating Brewery Effluent

The performance of a full-scale upflow anaerobic sludge blanket (UASB) reactor treating brewery wastewater was investigated by microbial analysis and kinetic modelling. The microbial community present in the granular sludge was detected using fluorescent in situ hybridization (FISH) and further confirmed using polymerase chain reaction. A group of 16S rRNA based fluorescent probes and primers targeting Archaea and Eubacteria were selected for microbial analysis. FISH results indicated the presence and dominance of a significant amount of Eubacteria and diverse group of methanogenic Archaea belonging to the order Methanococcales, Methanobacteriales, and Methanomicrobiales within in the UASB reactor. The influent brewery wastewater had a relatively high amount of volatile fatty acids chemical oxygen demand (COD), 2005 mg/l and the final COD concentration of the reactor was 457 mg/l. The biogas analysis showed 60–69 % of methane, confirming the presence and activities of methanogens within the reactor. Biokinetics of the degradable organic substrate present in the brewery wastewater was further explored using Stover and Kincannon kinetic model, with the aim of predicting the final effluent quality. The maximum utilization rate constant U _max and the saturation constant (K _B) in the model were estimated as 18.51 and 13.64 g/l/day, respectively. The model showed an excellent fit between the predicted and the observed effluent COD concentrations. Applicability of this model to predict the effluent quality of the UASB reactor treating brewery wastewater was evident from the regression analysis (R ²?=?0.957) which could be used for optimizing the reactor performance.     

Production of hydrogen from domestic wastewater in a pilot-scale microbial electrolysis cell

Addressing the need to recover energy from the treatment of domestic wastewater, a 120-L microbial electrolysis cell was operated on site in Northern England, using raw domestic wastewater to produce virtually pure hydrogen gas (100?±?6.4 %) for a period of over 3 months. The volumetric loading rate was 0.14 kg of chemical oxygen demand (COD) per cubic metre per day, just below the typical loading rates for activated sludge of 0.2–2 kg?COD?m^?3?day^?1, at an energetic cost of 2.3 kJ/g?COD, which is below the values for activated sludge 2.5–7.2 kJ/g?COD. The reactor produced an equivalent of 0.015 L?H₂?L^?1?day^?1, and recovered around 70 % of the electrical energy input with a coulombic efficiency of 55 %. Although the reactor did not reach the breakeven point of 100 % electrical energy recovery and COD removal was limited, improved hydrogen capture and reactor design could increase the performance levels substantially. Importantly, for the first time, a ‘proof of concept’ has been made, showing that this technology is capable of energy capture as hydrogen gas from low strength domestic wastewaters at ambient temperatures.     

Treatment of domestic wastewater in an up-flow anaerobic sludge blanket reactor followed by moving bed biofilm reactor

The performance of a laboratory-scale sewage treatment system composed of an up-flow anaerobic sludge blanket (UASB) reactor and a moving bed biofilm reactor (MBBR) at a temperature of (22–35 °C) was evaluated. The entire treatment system was operated at different hydraulic retention times (HRT’s) of 13.3, 10 and 5.0 h. An overall reduction of 80–86% for COD_total; 51–73% for COD_colloidal and 20–55% for COD_soluble was found at a total HRT of 5–10 h, respectively. By prolonging the HRT to 13.3 h, the removal efficiencies of COD_total, COD_colloidal and COD_soluble increased up to 92, 89 and 80%, respectively. However, the removal efficiency of COD_suspended in the combined system remained unaffected when increasing the total HRT from 5 to 10 h and from 10 to 13.3 h. This indicates that, the removal of COD_suspended was independent on the imposed HRT. Ammonia-nitrogen removal in MBBR treating UASB reactor effluent was significantly influenced by organic loading rate (OLR). 62% of ammonia was eliminated at OLR of 4.6 g COD m⁻² day⁻¹. The removal efficiency was decreased by a value of 34 and 43% at a higher OLR’s of 7.4 and 17.8 g COD m⁻² day⁻¹, respectively. The mean overall residual counts of faecal coliform in the final effluent were 8.9 × 10⁴ MPN per 100 ml at a HRT of 13.3 h, 4.9 × 10⁵ MPN per 100 ml at a HRT of 10 h and 9.4 × 10⁵ MPN per 100 ml at a HRT of 5.0 h, corresponding to overall log₁₀ reduction of 2.3, 1.4 and 0.7, respectively. The discharged sludge from UASB–MBBR exerts an excellent settling property. Moreover, the mean value of the net sludge yield was only 6% in UASB reactor and 7% in the MBBR of the total influent COD at a total HRT of 13.3 h. Accordingly, the use of the combined UASB–MBBR system for sewage treatment is recommended at a total HRT of 13.3 h.     

A one dimensional moving bed biofilm reactor model for nitrification of municipal wastewaters

This work presents a one-dimensional model of a moving bed bioreactor (MBBR) process designed for the removal of nitrogen from raw wastewaters. A comprehensive experimental strategy was deployed at a semi-industrial pilot-scale plant fed with a municipal wastewater operated at 10–12 °C, and surface loading rates of 1–2 g filtered COD/m² d and 0.4–0.55 g NH₄-N/m² d. Data were collected on influent/effluent composition, and on measurement of key variables or parameters (biofilm mass and maximal thickness, thickness of the limit liquid layer, maximal nitrification rate, oxygen mass transfer coefficient). Based on time-course variations in these variables, the MBBR model was calibrated at two time-scales and magnitudes of dynamic conditions, i.e., short-term (4 days) calibration under dynamic conditions and long-term (33 days) calibration, and for three types of carriers. A set of parameters suitable for the conditions was proposed, and the calibrated parameter set is able to simulate the time-course change of nitrogen forms in the effluent of the MBBR tanks, under the tested operated conditions. Parameters linked to diffusion had a strong influence on how robustly the model is able to accurately reproduce time-course changes in effluent quality. Then the model was used to optimize the operations of MBBR layout. It was shown that the main optimization track consists of the limitation of the aeration supply without changing the overall performance of the process. Further work would investigate the influence of the hydrodynamic conditions onto the thickness of the limit liquid layer and the “apparent” diffusion coefficient in the biofilm parameters.

     

Algae-based biofilm productivity utilizing dairy wastewater: effects of temperature and organic carbon concentration

Background

Biofilm-based microalgal growth was determined as functions of organic chemical loading and water temperature utilizing dairy wastewater from a full-scale dairy farm. The dairy industry is a significant source of wastewater worldwide that could provide an inexpensive and nutrient rich feedstock for the cultivation of algae biomass for use in downstream processing of animal feed and aquaculture applications. Algal biomass was cultivated using a Rotating Algal Biofilm Reactor (RABR) system. The RABR is a biofilm-based technology that has been designed and used to remediate municipal wastewater and was applied to treat dairy wastewater through nutrient uptake, and simultaneously provide biomass for the production of renewable bioproducts.

Results

Aerial algal biofilm growth rates in dairy wastewater at 7 and 27 °C temperatures were shown to be 4.55?±?0.17 g/m²-day and 7.57?±?1.12 g/m²-day ash free dry weight (AFDW), respectively. Analysis of Variance (ANOVA) calculations indicated that both an increase in temperature of the wastewater and an increase in the level of organic carbon, from 300 to 1200 mg L^-1, contributed significantly to an increase in the rate of biomass growth in the system. However, ANOVA results indicated that the interaction of temperature and organic carbon content was not significantly related to the biofilm-based growth rate.

Conclusion

A microalgae-based biofilm reactor was successfully used to treat turbid dairy wastewater. Temperature and organic carbon concentration had a statistically significant effect on algae-based biofilm productivity and treatment of dairy wastewater. The relationships between temperature, TOC, and productivity developed in this study may be used in the design and assessment of wastewater remediation systems and biomass production systems utilizing algae-based biofilm reactors for treating dairy wastes.

     

Coupling microbial fuel cells with a membrane photobioreactor for wastewater treatment and bioenergy production

Microbial fuel cells (MFCs) and membrane photobioreactors are two emerging technologies for simultaneous wastewater treatment and bioenergy production. In this study, those two technologies were coupled to form an integrated treatment system, whose performance was examined under different operating conditions. The coupled system could achieve 92–97 % removal of soluble chemical oxygen demand (SCOD) and nearly 100 % removal of ammonia. Extending the hydraulic retention time (HRT) of the membrane photobioreactor to 3.0 days improved the production of algal biomass from 44.4 ± 23.8 to 133.7 ± 12.9 mg L^?1 (based on the volume of the treated water). When the MFCs were operated in a loop mode, their effluent (which was the influent to the algal reactor) contained nitrate and had a high pH, leading to the decreased algal production in the membrane photobioreactor. Energy analysis showed that the energy consumption was mainly due to the recirculation of the anolyte and the catholyte in the MFCs and that decreasing the recirculation rates could significantly reduce energy consumption. The energy production was dominated by indirect electricity generation from algal biomass. The highest energy production of 0.205 kWh m^?3 was obtained with the highest algal biomass production, resulting in a theoretically positive energy balance of 0.033 kWh m^?3. Those results have demonstrated that the coupled system could be an alternative approach for energy-efficient wastewater treatment and using wastewater effluent for algal production.     

Applicability of one-stage partial nitritation and anammox in MBBR for anaerobically pre-treated municipal wastewater

Energy consumption of municipal wastewater treatment plants can be reduced by the anaerobic pre-treatment of the main wastewater stream. After this pre-treatment, nitrogen can potentially be removed by partial nitritation and anammox (PN/A). Currently, the application of PN/A is limited to nitrogen-rich streams (>500 mg L^?1) and temperatures 25–35 °C. But, anaerobically pretreated municipal wastewater is characterized by much lower nitrogen concentrations (20–100 mg L^?1) and lower temperatures (10–25 °C). We operated PN/A under similar conditions: total ammonium nitrogen concentration 50 mg L^?1 and lab temperature (22 °C). PN/A was operated for 342 days in a 4 L moving bed biofilm reactor (MBBR). At 0.4 mg O₂ L^?1, nitrogen removal rate 33 g N m^?3 day^?1 and 80 % total nitrogen removal efficiency was achieved. The capacity of the reactor was limited by low AOB activity. We observed significant anammox activity (40 g N m^?3 day^?1) even at 12 °C, improving the applicability of PN/A for municipal wastewater treatment.     

Continuous fill intermittent decant type sequencing batch reactor application to upgrade the UASB treated sewage

The performance of continuous flow intermittent decant type sequencing batch (CFID) reactor treating the effluent of an UASB reactor treating domestic wastewater and operated at 8 h hydraulic retention time (HRT) was investigated. The CFID was operated at three different HRTs (22, 8 and 6 h) and three different dissolved oxygen (DO) patterns (<0.5, 2.5–3.5 and 3.5–4.5 mg/L). The highest effluent quality was observed at the 8 h HRT and 2.5–3.5 mg/L DO concentration. At this operational condition, the average BOD, TSS, ammonia nitrogen and fecal coliform removal efficiencies were 83, 90, 74 and 99 %, respectively. The CFID is a promising post-treatment option for existing UASB systems, with a final effluent quality that comply with receiving water and effluent reuse criteria.     

Development of a fixed-bed anammox reactor with high treatment potential

A plug-flow type anaerobic ammonium oxidation (anammox) reactor was developed using malt ceramics (MC) produced from carbonized spent grains as the biomass carriers for anammox sludge. Partial nitrified effluent of the filtrate from the sludge dehydrator of a brewery company was used as influent to a 20 L anammox reactor using MC. An average volumetric nitrogen removal rate (VNR) of 8.78 kg-N/m³/day was maintained stably for 76 days with 1 h of HRT. In a larger anammox reactor (400 L), an average VNR of 4.84 kg-N/m³/day could be maintained for 86 days during the treatment of low strength synthetic inorganic wastewater. As a result of bacterial community analysis for the 20 L anammox reactor, Asahi BRW1, probably originating from the wastewater collected at Asahi Breweries, was detected as the dominant anammox bacterium. These anammox reactors were characterized by a high NH₄-N removal capacity for low strength wastewater with a short hydraulic retention time.     

Process contribution evaluation for COD removal and energy production from molasses wastewater in a BioH<Subscript>2</Subscript>–BioCH<Subscript>4</Subscript>–MFC-integrated system

In this study, a three-stage-integrated process using the hydrogenic process (BioH₂), methanogenic process (BioCH₄), and a microbial fuel cell (MFC) was operated using molasses wastewater. The contribution of individual processes to chemical oxygen demand (COD) removal and energy production was evaluated. The three-stage integration system was operated at molasses of 20 g-COD L^?1, and each process achieved hydrogen production rate of 1.1 ± 0.24 L-H₂ L^?1 day^?1, methane production rate of 311 ± 18.94 mL-CH₄ L^?1 day^?1, and production rate per electrode surface area of 10.8 ± 1.4 g m^?2 day^?1. The three-stage integration system generated energy production of 32.32 kJ g-COD^?1 and achieved COD removal of 98 %. The contribution of BioH₂, BioCH₄, and the MFC reactor was 20.8, 72.2, and, 7.0 % of the total COD removal, and 18.7, 81.2, and 0.16 % of the total energy production, respectively. The continuous stirred-tank reactor BioH₂ at HRT of 1 day, up-flow anaerobic sludge blanket BioCH₄ at HRT of 2 days, and MFC reactor at HRT of 3 days were decided in 1:2:3 ratios of working volume under hydraulic retention time consideration. This integration system can be applied to various configurations depending on target wastewater inputs, and it is expected to enhance energy recovery and reduce environmental impact of the final effluent.     

Domestic wastewater treatment using multi-electrode continuous flow MFCs with a separator electrode assembly design

Treatment of domestic wastewater using microbial fuel cells (MFCs) will require reactors with multiple electrodes, but this presents unique challenges under continuous flow conditions due to large changes in the chemical oxygen demand (COD) concentration within the reactor. Domestic wastewater treatment was examined using a single-chamber MFC (130 mL) with multiple graphite fiber brush anodes wired together and a single air cathode (cathode specific area of 27 m²/m³). In fed-batch operation, where the COD concentration was spatially uniform in the reactor but changed over time, the maximum current density was 148?±?8 mA/m² (1,000 Ω), the maximum power density was 120 mW/m², and the overall COD removal was >90 %. However, in continuous flow operation (8 h hydraulic retention time, HRT), there was a 57 % change in the COD concentration across the reactor (influent versus effluent) and the current density was only 20?±?13 mA/m². Two approaches were used to increase performance under continuous flow conditions. First, the anodes were separately wired to the cathode, which increased the current density to 55?±?15 mA/m². Second, two MFCs were hydraulically connected in series (each with half the original HRT) to avoid large changes in COD among the anodes in the same reactor. The second approach improved current density to 73?±?13 mA/m². These results show that current generation from wastewaters in MFCs with multiple anodes, under continuous flow conditions, can be improved using multiple reactors in series, as this minimizes changes in COD in each reactor.     

Biofilm producing indigenous bacteria isolated from municipal sludge and their nutrient removal ability in moving bed biofilm reactor from the wastewater

In the present study, improved moving bed biofilm reactor (MBBR) was applied to enhance the nutrient removal ability of the municipal wastewater. A total of 18 indigenous bacterial isolates were screened from the sewage sludge sample and nitrate reductase, nitrite reductase and hydroxylamine oxidase was analyzed. The strains Pseudomonas aeruginosa NU1 and Acinetobacter calcoaceticus K12 produced 0.87 ± 0.05 U/mg and 0.52 ± 0.12 U/mg hydroxylamine oxidase, 1.023 ± 0.062 U/mg and 1.29 ± 0.07 U/mg nitrite reductase, and 0.789 ± 0.031 U/mg and 1.07 ± 0.13 U/mg nitrate reductase. Nitrogen and phosphate removal improved by the addition of nutrient sources and achieved > 80% removal rate. pH and temperature of the medium also affected nutrient removal and improved removal was achieved at optimum level (p < 0.05). MBBR was designed with R1 (aerobic), R2 and R3 (anoxic) reactors. MBBR reactors removed acceptable level phosphorus removal properties up to 7.2 ± 3.8%, 42.4 ± 4.6%, and 84.2 ± 13.1% in the R1, R2, R3 and R4 reactors, respectively. Denitrification rate showed linear relationship at increasing concentrations nitrogen content in the reactor and denitrification rate was 1.43 g NO₂-N /m²/day at 1.5 g NO₂-N /m²/day. Dehydrogenase activity was assayed in all reactors and maximum amount was detected in the aerobic biofilm reactor. Based on the present findings, MBBRs and the selected bacterial strains are useful for the degradation domestic wastewater with minimum working area.     

Enhanced remediation of black liquor by activated sludge bioaugmented with a novel exogenous microorganism culture

Black liquor (BL) is a notoriously difficult wastewater to treat due to the economic and efficiency limitations of physiochemical methods and intrinsic difficulties with bioremediation strategies caused by the high pH (10–13) and lignin content. This study investigated the feasibility of a novel bioaugmentation strategy for BL treatment, which uses a mixed microorganism culture of lignocellulose-degrading microorganisms isolated from degraded bamboo slips. Black liquor treatment was assessed in terms of chemical oxygen demand (COD) and color removal with a sequencing batch reactor organic loading rate of 9 kg COD/L·day under highly alkaline conditions (pH?10). Results revealed that bioaugmented activated sludge treatment of BL with special mixed microorganisms significantly enhanced the removal efficiency of COD, color, and lignin from the wastewater up to 64.8, 50.5, and 53.2 %, respectively. Gel permeation chromatography profiles showed that the bioaugmentation system could successfully degrade high molecular lignin fragments in black liquor. This work confirms bioaugmentation as a feasible alternative strategy for enhanced biological treatment of wastewater with high lignin content and high organic load rate under strongly alkaline conditions.     

Microflora from leaf debris is suitable for treatment of starch industry wastewater

Biological treatment of industrial waste is a widely practiced technique that generates comparatively less environmentally hazardous waste than other chemical treatment processes. Wet milling of maize generates huge amount of wastewater (5 m³/ton) of low pH with organic matter and nutrients. Anaerobic methanogenic and aerobic bacteria are mostly highly sensitive to low pH. The treatment of wastewater causes huge cost of chemical neutralization or hydraulic recirculation for maintaining neutral pH. In the present study, different microbial consortia isolated from cow dung, active sludge from an anaerobic reactor for treatment of industrial wastewater, and leaf debris from benthic soil were screened for tolerance against low pH and for potential of chemical oxygen demand (COD) removal in order to find out an alternative microbial population for industrial water treatment at low pH. The most effective consortia found from leaf debris were further investigated for optimal operation. The microscopic analysis of leaf debris sludge showed abundance of Gram‐negative methanococci, which was found tolerant to low pH in plate culture method. On further investigation for COD removal from starch industry effluent, they were found to be most effective at pH 5 with highest COD removal rate of 70% and lowest biomass generation of 81%. Hence, it was concluded that the low pH‐tolerant methanogen bacteria, enriched from leaf debris sludge, is highly beneficial for anaerobic treatment of wastewater from several industries including corn starch industry by reducing cost of operation for neutralization to neutral pH and through reducing excess waste sludge production by the treatment system.     

Pilot-scale experiment of down-flow hanging sponge for direct treatment of low-strength municipal wastewater in Bangkok,Thailand

A pilot-scale experiment of a down-flow hanging sponge (DHS) reactor for treatment of low-strength municipal wastewater was conducted over 1 year in Bangkok, Thailand, to establish an appropriate method for treatment under tropical climate conditions. Municipal wastewater with an average BOD of 19 mg/L was fed directly into the DHS reactor. Superior effluent quality (5.1 ± 3.4 mg/L TSS, 21.1 ± 9.0 mg/L COD, 2.8 ± 1.4 mg/L BOD, and 4.1 ± 1.0 mg/L TN) was achieved at a hydraulic retention time (HRT) of 1 h under an average temperature of 30 °C. The DHS reactor reached an actual HRT of 19.0 min, indicating good contact efficiency between wastewater and retained sludge. The DHS reactor retained dense sludge at 15.3–26.4 g VSS/L based on the sponge media volume. The sludge activity in terms of specific oxygen uptake rate was good. Excess sludge was produced as 0.051 g TSS/g COD removed (0.11 g TSS/g BOD removed), and a good SVI of 28 mL/g was observed. The sufficient performance was attributed to dense sludge with high activity, regardless of the low-strength wastewater. Overall, the DHS was advantageous owing to its simple operation, lack of operational problems, and low power consumption.     

Partial nitrification and nutrient removal in intermittently aerated sequencing batch reactors treating separated digestate liquid after anaerobic digestion of pig manure

The performance of an intermittently aerated sequencing batch reactor (IASBR) technology was investigated in achieving partial nitrification, organic matter removal and nitrogen removal from separated digestate liquid after anaerobic digestion of pig manure. The wastewater had chemical oxygen demand (COD) concentrations of 11,540 ± 860 mg/L, 5-day biochemical oxygen demand (BOD₅) concentrations of 2,900 ± 200 mg/L and total nitrogen (TN) concentrations of 4,041 ± 59 mg/L, with low COD:N ratios (2.9) and BOD₅:COD ratios (0.25). Synthetic wastewater, simulating the separated digestate liquid with similar COD and nitrogen concentrations but BOD₅ of 11,500 ± 100 mg/L, was also treated using the IASBR technology. At a mean organic loading rate of 1.15 kg COD/(m³ d) and a nitrogen loading rate of 0.38 kg N/(m³ d), the COD removal efficiency was 89.8% in the IASBR (IASBR-1) treating digestate liquid and 99% in the IASBR (IASBR-2) treating synthetic wastewater. The IASBR-1 effluent COD was mainly due to inert organic matter and can be further reduced to less than 40 mg/L through coagulation. The partial nitrification efficiency of 71–79% was achieved in the two IASBRs and one cause for the stable long-term partial nitrification was the intermittent aeration strategy. Nitrogen removal efficiencies were 76.5 and 97% in IASBR-1 and IASBR-2, respectively. The high nitrogen removal efficiencies show that the IASBR technology is a promising technology for nitrogen removal from low COD:N ratio wastewaters. The nitrogen balance analysis shows that 59.4 and 74.3% of nitrogen removed was via heterotrophic denitrification in the non-aeration periods in IASBR-1 and IASBR-2, respectively.     

Winery wastewater treatment: a critical overview of advanced biological processes

Wine production is one of the leading sectors of the food processing industry. The wine industry produces a large amount of wastewater characterized by a high strength in terms of organic pollution and large variability throughout the year. Most of the organic matter is soluble and easily biodegradable. On the other hand, nitrogen and phosphorous are lacking. The aerobic and anaerobic processes are largely applied for winery wastewater treatment because they can quickly react to changes in the organic loading. This review analyzes e applied biological systems, considering both aerobic and anaerobic processes, and different reactor configurations. The performances of different biological processes are evaluated in terms of operational conditions (organic loading rate and hydraulic retention time). Aerobic processes can guarantee chemical oxygen demand removal up to 98% for organic loading rates of some 1-2?kg of chemical oxygen demand m^?3d^?1 but requires good aeration systems to supply the required process oxygen. The management cost of these processes could be high considering the power density in the range 60-70?W m^?3_reactor and that nutrients should be added to support biomass growth. On the other hand, anaerobic processes are able to face high organic loads with low running costs, but COD removal is generally limited to 90%. Combination of the two treatment systems (anaerobic followed by aerobic) could reduce management costs and meet high discharge standards.