Chemically Enhanced Primary Sedimentation Research Articles

Organic binding iron formation and its mitigation in cation exchange resin assisted anaerobic digestion of chemically enhanced primary sedimentation sludge

Fe based chemically enhanced primary sedimentation (CEPS) is an effective method of capturing the colloidal particles and inorganic phosphorous (P) from wastewater but also produces Fe-CEPS sludge. Anaerobic digestion is recommended to treat the sludge for energy and phosphorus recovery. However, the aggregated sludge flocs caused by the coagulation limited sludge hydrolysis and P release during anaerobic digestion process. In this study, cation exchange resin (CER) was employed during anaerobic digestion of Fe-CEPS sludge with aims of prompting P release and carbon recovery. CER addition effectively dispersed the sludge flocs. However, the greater dispersion of sludge flocs could not translate to higher sludge hydrolysis. The maximum hydrolysis and acidification achieved at lower CER dosage of 0.5 g CER/g TS. It was observed that the extents of sludge hydrolysis and acidification had a strongly negative correlation with the organic binding iron (OBI) concentration. The presence of CER during anaerobic digestion favored Fe(III) reduction to Fe(II), and then further induced iron phase transformation, leading to the OBI formation from the released organic matters. Meanwhile, higher CER dosage resulted in higher P release efficiency and the maximum efficiency at 4 g CER/g TS was four times than that of the control. The reduction of BD-P, NaOH-P and HCl-P in solid phase contributed most P release into the supernatant. A new two-stage treatment process was further developed to immigrate the OBI formation and improve the carbon recovery efficiency. Through this process, approximately 45% of P was released, and 63% of carbon was recovered as methane from Fe-CEPS sludge via CER pretreatment.

Iron-enhanced primary sedimentation and acidogenic sludge fermentation to achieve self-sufficient organic carbon supply for enhanced nutrient removal in wastewater treatment✰

To treat municipal wastewater with a low carbon to nitrogen ratio, the addition of an external carbon source is often needed to achieve enhanced nutrient (nitrogen and phosphorus) removal. To meet this need, this research developed an innovative strategy for reserving and utilizing organic pollutants to achieve a self-sufficient organic carbon supply for wastewater treatment. In the new system, before biological treatment by a sequencing batch reactor (SBR), Fe-based chemically enhanced primary sedimentation (CEPS) was used to reserve part of the organics from the wastewater in the CEPS sludge. By acidogenic fermentation in the side-stream, the settled organics in the CEPS sludge were converted into volatile fatty acids and other soluble organics to provide a suitable carbon source for denitrification in the anoxic phase of the SBR. The experimental results showed that the new SBR system integrated with CEPS and acidogenic sludge fermentation removed up to 89% of the total phosphorus (TP) and 83% of the total nitrogen (TN) from the wastewater. Compared with the conventional SBR system, TP and TN removal was increased by more than 65% and 50%, respectively. As well as increasing nutrient removal efficiency without the need for external carbon addition, the Fe-based CEPS greatly reduced the organic load on the SBR biological treatment process, reducing the aeration cost. Overall, a cost-effective wastewater treatment process is developed that can be used to upgrade existing wastewater treatment plants to improve the nutrient removal performance and meet more stringent discharge standards.

Production of carbon-doped titanium dioxide (C–TiO2) from polytitanium-coagulated sludge as an adsorbent or photocatalyst for pollutant removals

Chemically enhanced primary sedimentation (CEPS) using titanium coagulants could produce the Ti-loaded sludge with promising material recycle and utilization options. This study investigated two approaches for the recovery and utilization of Ti-based CEPS sludge: (1) a current technology of sludge calcination in the absence of N2 atmosphere to transform Ti to carbon-doped TiO2 (C–TiO2, with a low C content) as a photocatalyst, and (2) a prospective technology of carbonization of the sludge in the presence of N2 atmosphere to produce C–TiO2 (high C content) as an adsorbent. Both approaches can be applied for the recycle and utilization of the materials of the Ti-based CEPS sludge. Calcination at 400–800 °C led to the predominant anatase-TiO2 formation (spherical granules with particle size of around 20–25 nm), while rutile crystal phase appeared at 900 °C. The as-obtained C–TiO2 performed effectively for the adsorption or photocatalytic degradation of rhodamine B (RB) and methyl orange (MO), as the model pollutants, in wastewater treatment. The C–TiO2 with rich activated carbon content (20.0–38.3%, functioned as adsorbents) achieved high-efficient RB and MO adsorption by up to 98% in 20 min, while the C–TiO2 with low activated carbon content (1.5–12.6%, functioned as photocatalysts) degraded 99.6% of RB and 97.0% of MO from initial 20 mg/L in 210 min by photocatalytic degradation under UV illumination.

Chemically enhanced primary sedimentation and acidogenesis of organics in sludge for enhanced nitrogen removal in wastewater treatment

Abstract Ferric iron-based chemically enhanced primary sedimentation (CEPS) can remove over 70% of organics from municipal wastewater into sludge, and acidogenic fermentation converts 27.9% of solid organic substances in the CEPS sludge into soluble ones, mostly in the form of volatile fatty acids. In this study, the CEPS sludge supernatant after fermentation (CEPS-FL) was studied as an organic carbon source for biological denitrification, in comparison with the common external carbon sources of methanol (CH3OH), sodium acetate (NaAc), and sodium propionate (NaPr). The specific denitrification rates of NaAc, NaPr, and CEPS-FL were 701.3, 449.8, and 410.3 g N·kg VSS−1·d−1, respectively, obviously higher than the value of CH3OH (237.2 g N·kg VSS−1·d−1). During heterotrophic denitrification, lower N2O yields were observed in VFAs-based denitrification reactors (0.53% of NaAc, 1.11% of NaPr, and 1.14% of CEPS-FL) than in the reactor fed with CH3OH (3.42%). The bacterial community of the denitrification sludge was significantly affected by the type of carbon sources. The sludge fed with CEPS-FL had a greater alpha diversity than that fed with pure organic carbons, with the dominance of Zoogloea (14.7%), Alicycliphilus (7.6%), and Thauera (6.2%). Based on the mass balance of CEPS-fermentation-denitrification combination, the ammonium-nitrogen in the CEPS-FL would not affect the overall nitrogen removal performance. In comparison with the municipal wastewater treatment supplemented with CH3OH or NaAc as the external carbon source for improved nitrogen removal, the operating cost of this combined system can be considerably lower. Overall, a small modification in primary sedimentation by CEPS in combination with the sludge fermentation would not only decrease the organic load on the subsequent biological treatment and significantly improve the nitrogen removal performance, but also provide an effective, chemical-saving, and energy-efficient way for wastewater treatment.

Fe-enhanced primary sedimentation, acidogenic sludge fermentation and electrodialysis for resource recovery from domestic wastewater

In this study, a new chemical-biological process has been developed for municipal wastewater treatment. The new process utilises iron-based chemically enhanced primary sedimentation (CEPS) to concentrate organics and phosphorus (P) into sludge and hence reduces the load of pollutants on the downstream treatment process. A side-steam module for acidogenic sludge fermentation was applied to convert wastes in CEPS sludge to valuable resources. Results showed that over 70% of total organic carbon (TOC) and 83% of total phosphorus (TP) were removed by CEPS with an addition of 20 mg-Fe/L of FeCl3 while the total nitrogen (TN) removal efficiency was limited. Acidogenic fermentation of Fe-sludge under the mesophilic condition produced volatile fatty acids (VFAs) at a ratio of 137.7 mg-C/g-volatile solids (VS) with 12% of TP dissolution. With 1 g/L of starch addition for co-fermentation, the VFAs yield and P dissolution increased significantly to 201.3 mg-C/g-VS and 60%, respectively. However, the addition of peptones did not improve the P dissolution, due to the higher ammonium release that prevented the pH drop and acidification in the fermenter. Through the electrodialysis (ED) treatment in a five-chamber cell, 50% of ammonium in the fermented sludge liquor was concentrated into catholyte, while 35%-43% of phosphate, acetate (Ac) and propionate (Pr) were separated into anolyte.

Performance and bacterial community of moving bed biofilm reactors with various biocarriers treating primary wastewater effluent with a low organic strength and low C/N ratio

A laboratory-scale sequencing batch reactor (SBR) and two moving bed biofilm reactors (MBBRs) with different types of biocarriers were operated to treat the effluent of chemically enhanced primary sedimentation (CEPS). Due to the low organic strength and low carbon/nitrogen ratio of the CEPS effluent, COD and NH4+-N were effectively removed by the MBBRs but not by the SBR. Of the two MBBRs, MBBR2 filled with LEVAPOR biocarrier cubes performed even better than MBBR1 filled with K3 polystyrene biocarriers. The continuous decline of the sludge concentration in the SBR and the high and stable biomass content in MBBR2 contributed to their performances. High-throughput sequencing analysis showed that the reactors had selective effects on the bacterial community structure. Principal coordinate analysis indicated the different dynamic successions in the three reactors. Network analysis showed different community composition and diversity that were highly suggestive of different bacterial interactions among the three bioreactors.

Electro-fermentation of iron-enhanced primary sedimentation sludge in a two-chamber bioreactor for product separation and resource recovery

Iron (Fe)-based chemically enhanced primary sedimentation (CEPS) is an effective process used to remove organic and phosphorus (P) pollutants from wastewater into sludge. In this study, electro-fermentation (EF) technology was developed to treat the organic- and P-rich Fe-sludge for the purposes of sludge reduction and resource recovery. Using a two-chamber bioreactor with a cation exchange membrane and an exterior voltage (0.5–1.0 V) for sludge treatment, the EF system enabled product accumulation and separation during the fermentation process. Compared with the conventional fermentation in a single-chamber reactor, the EF treatment of Fe-sludge significantly improved the efficiency of P dissolution from the sludge from 8% to 56% after 4 d. Meanwhile, about 70% of released ammonium ions and 50% of Fe in the sludge were driven by the current from the sludge suspension into the cathode chamber for potential recovery. With a similar yield of volatile fatty acids (VFAs) but less ammonium remaining, the CODVFA/NNH4 of the sludge supernatant from the EF reactor could reach 67.7, much higher than that from the single-chamber fermenter at 13.0. The protease activity was effectively promoted by the EF treatment, suggesting that both electrochemical and biological hydrolysis effects contributed to the increased P release from the sludge. Microbial community analysis showed that the electrical stimulation increased the relative abundance of Firmicutes and facilitated the growth of Acidobacteria. Overall, electro-fermentation was developed as an effective biotechnology for processing Fe-based CEPS sludge with the benefits of recovering organics, phosphorus, ammonium and iron resources.

Phosphorus release and recovery from Fe-enhanced primary sedimentation sludge via alkaline fermentation

Phosphorus release and recovery from Fe-based chemically enhanced primary sedimentation (CEPS) sludge via alkaline fermentation was investigated. The coagulation results showed that 78% of organic matter and 95% of phosphorus were concentrated from sewage into sludge with the optimum dosages of 25 mg/L FeCl3. The batch fermentation results revealed that 69.35% of the phosphorus in the Fe-sludge can be released and the maximum phosphorus concentration was 20.57 mg/L at pH 11. In the recovery stage, 90% of the P released in the fermented sludge supernatant was precipitated at a 2:1 ratio of magnesium to phosphorus and pH 11. The result of X-ray diffraction indicated that magnesium ammonium phosphate (MAP) was the major component of the precipitated solids. Thus, the present study provides an alternative option for phosphorus release and recovery as MAP from CEPS sludge via alkaline fermentation.

Fermentation liquor of CaO2 treated chemically enhanced primary sedimentation (CEPS) sludge for bioplastic biosynthesis

Chemically enhanced primary sedimentation (CEPS) technology has been widely applied in Hong Kong, exhibiting excellent performance in contaminants removal from sewage. The generated CEPS sludge contains abundance of organics which could be recovered as volatile fatty acids (VFAs) by fermentation for further utilization. In this work, the effect of calcium peroxide (CaO2) on the fermentation of FeCl3 based CEPS sludge was investigated. The feasibility of utilizing the fermentation liquor as substrate for polyhydroxyalkanoates (PHAs) biosynthesis was also evaluated. Results demonstrated that CaO2 addition facilitated the disintegration of CEPS sludge and enhanced VFAs production. The maximum VFAs yield of 455.8 mg COD/g VSS was obtained with the dosage of 0.1 g CaO2/g SS, improving by 44.7% compared with the control sludge. Acetic and propionic acid were the predominant components of the VFAs. Microbial analysis indicated that CaO2 induced microbial reduction of Fe(III), accelerating the initial disintegration of FeCl3 based CEPS sludge. Microbial communities with hydrolysis and acidogenesis functions were enriched effectively. CaO2 treatment had no significant influence on the release of ammonia nitrogen (NH4+-N), while reduced the concentration of orthophosphate (PO43−-P) and ferrous (Fe2+) in fermentation liquor, that was beneficial to the further utilization as substrate for PHAs biosynthesis. The VFA-rich fermentation liquor was proved to be a suitable substrate for PHAs biosynthesis. After cultivation, the PHAs content in activated sludge reached 22.3%, which was comparable to those obtained using waste materials as carbon source. This integrated technology could be a superior alternative of realizing sludge disposal and bioplastic production simultaneously.

Acidogenic sludge fermentation to recover soluble organics as the carbon source for denitrification in wastewater treatment: Comparison of sludge types

For biological nitrogen (N) removal from wastewater, a sufficient organic carbon source is requested for denitrification. However, the organic carbon/nitrogen ratio in municipal wastewater is becoming lower in recent years, which increases the demand for the addition of external organic carbon, e.g. methanol, in wastewater treatment. The volatile fatty acids (VFAs) produced by acidogenic fermentation of sewage sludge can be an attractive alternative for methanol. Chemically enhanced primary sedimentation (CEPS) is an effective process that applies chemical coagulants to enhance the removal of organic pollutants and phosphorus from wastewater by sedimentation. In terms of the chemical and biological characteristics, the CEPS sludge is considerably different from the conventional primary and secondary sludge. In the present study, FeCl3 and PACl (polyaluminum chloride) were used as the coagulants for CEPS treatment of raw sewage. The derived CEPS sludge (Fe-sludge and Al-sludge) was then processed with mesophilic acidogenic fermentation to hydrolyse the solid organics and produce VFAs for organic carbon recovery, and the sludge acidogenesis efficiency was compared with that of the conventional primary sludge and secondary sludge. The results showed that the Fe-sludge exhibited the highest hydrolysis and acidogenesis efficiency, while the Al-sludge and secondary sludge had lower hydrolysis efficiency than that of primary sludge. Utilizing the Fe-sludge fermentation liquid as the carbon source for denitrification, more than 99% of nitrate removal was achieved in the main-stream wastewater treatment without any external carbon addition, instead of 35% obtained from the conventional process of primary sedimentation followed by the oxic/anoxic (O/A) treatment.

Acidogenic fermentation of iron-enhanced primary sedimentation sludge under different pH conditions for production of volatile fatty acids

Iron-based chemically enhanced primary sedimentation (CEPS) is increasingly adopted for wastewater treatment in mega cities, producing a large amount of sludge (Fe-sludge) with a high content of organics for potential organic resource recovery. In this experimental study, acidogenic fermentation was applied treat FeCl3-based CEPS sludge for production of volatile fatty acids (VFAs) at different pHs. Batch fermentation tests on the Fe-sludge with an organic content of 10 g-COD/L showed that the maximum VFAs production reached 2782.2 mg-COD/L in the reactor without pH control, and it reached 688.4, 3095.3, and 2603.7 mg-COD/L in reactors with pHs kept at 5.0, 6.0 and 8.0, respectively. Analysis of the acidogenesis kinetics and enzymatic activity indicated that the alkaline pH could accelerate the rate of organic hydrolysis but inhibited the further organic conversion to VFAs. In semi-continuous sludge fermentation tests, the VFAs yield in the pH6 reactor was 20% higher than that in the control reactor without pH regulation, while the VFAs yield in the pH8 reactor was 10% lower than the control. Illumina MiSeq sequencing revealed that key functional microorganisms known for effective sludge fermentation, including Bacteroidia and Erysipelotrichi, were enriched in the pH6 reactor with an enhanced VFAs production, while Clostridia became more abundant in the pH8 reactor to stand the unfavorable pH condition. The research presented acidogenic fermentation as an effective process for CEPS sludge treatment and organic resource recovery and provided the first insight into the related microbial community dynamics.

Recovery of organic resources from sewage sludge of Al-enhanced primary sedimentation by alkali pretreatment and acidogenic fermentation

Primary sludge from wastewater treatment is of rich organic contents that can be resource for recovery. However, the sludge derived from the chemically enhanced primary sedimentation (CEPS), especially using Al-based coagulants, features a rather low biodegradability and treatability due to the agglomeration of solid matters by the coagulant. In the present study, alkali pretreatments by three chemicals (NaHCO3, Na2CO3 and NaOH) were applied to disintegrate Al-based CEPS sludge (Al-sludge) aggregates and improve its hydrolysis and biodegradability for fermentation and organic recovery. Results showed that alkali pretreatment significantly reduced the Al-sludge particle size and accelerated the release of the enmeshed organics. NaHCO3 exhibited the best pretreatment result. At ambient temperature, the yield of volatile fatty acids (VFAs) of the Al-sludge pretreated by Na2CO3 (initial pH 10) was improved by four times compared to the untreated sludge, resulting from both the improved abiotic and biotic hydrolysis effects. The effect of alkali pretreatment on Al-sludge was found to be mainly attributable to the pH adjustment rather than the alkalinity increase. Illumina MiSeq sequencing revealed that protein-fermenting bacteria Treponema and Bacteroides became dominant during fermentation of the alkali pretreated sludge. By PACl-based CEPS and enhanced sludge acidogenesis, around 17% of organics in municipal wastewater can be recovered, mainly in the form of VFAs, which is significantly greater than 6% by the conventional primary sedimentation and sludge fermentation.

Effect of coagulant on acidogenic fermentation of sludge from enhanced primary sedimentation for resource recovery: Comparison between FeCl3 and PACl

FeCl3 and PACl as coagulants in chemically enhanced primary sedimentation (CEPS) were compared in terms of their efficiencies in removing pollutants from wastewater and their effects on the acidogenic fermentation of CEPS sludge for resource recovery. PACl was found to be more effective than FeCl3 for removing suspended solids by CEPS, with around 20% higher removal efficiency. However, the coagulated Al-sludge experienced more difficulty and had lower efficiency than Fe-sludge in organic hydrolysis and acidogenesis. The batch fermentation results showed that FeCl3 dosed at 10–30mg Fe/Lsewage had little influence on sludge hydrolysis and volatile fatty acid (VFA) production, whereas an obvious inhibitory effect was observed for PACl in organic hydrolysis of the sludge. The specific hydrolysis rate constant (Kh,P) for sludge fermentation decreased from 0.0321 for the sludge without PACl to 0.017 for the Al-sludge obtained at a dosage of 24mgAl/Lsewage. Compared to the Al-sludge, the Fe-sludge had a much higher VFA yield and significant PO4–P release during the sludge fermentation, which is attributed to the reduction of Fe(III) to Fe(II) under anaerobic conditions and the resulting disintegration of sludge flocs. By simple pH adjustment of the fermented Fe-sludge supernatant, up to 31% of the phosphorus in wastewater can be recovered in the form of vivianite as P fertilizers. VFAs produced in the supernatant are valuable organic carbon resources that can be recovered for beneficial uses.

Recovery of organic carbon and phosphorus from wastewater by Fe-enhanced primary sedimentation and sludge fermentation

Abstract A new chemically enhanced primary sedimentation (CEPS) and sludge fermentation process are developed for improved nutrient removal, energy saving and resource recovery in municipal wastewater treatment. The FeCl3-based CEPS with a dosage of 20 mg-Fe/L can remove 75.6% of organic pollutants and 99.3% of PO4-P on average from wastewater. Under natural fermentation conditions, the CEPS sludge undergoes effective hydrolysis and acidogenesis to produce volatile fatty acids (VFAs) and release phosphate as valuable resources. By using CEPS, around 27% of the organic carbon in wastewater influent can be recovered via sludge fermentation, mainly in the form of VFAs, and about 23% of phosphorus recovered for making vivianite fertilizer. In comparison, both the organic and phosphorus recovery ratios from wastewater are under 10% with conventional primary sedimentation and sludge fermentation. CEPS combined with side-stream sludge fermentation can be readily applied in new treatment plants or in a retrofit of existing treatment systems.

Study of microbial community structures in UASB sludge treating municipal wastewater by denaturing gradient gel electrophoresis of 16S rDNA

The structures of microbial communities in lab-scale upflow anaerobic sludge blanket (UASB) reactors for treating municipal wastewater with different ratios of COD soluble/COD total were studied using denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes. The microbial structure of the inoculum sludge obtained from a full-scale UASB reactor of treating potato processing wastewater was compared with the structures of sludges collected from three lab-scale UASB reactors after eight months feeding with raw municipal wastewater, with CEPS (chemically enhanced primary sedimentation) pretreated municipal wastewater, and with a synthetic municipal sewage, respectively. Computer-aided numerical analysis of the DGGE fingerprints showed that the bacterial community underwent major changes. The sludges for treating raw and CEPS pretreated wastewater had very similar bacterial and archaeal communities (82% and 96% similarity) but were different from that for treating the synthetic sewage. Hence, despite similar % COD in the particulate form in the synthetic and the real wastewater, the two wastewaters were selected for different microbial communities. Prominent DGGE bands of Bacteria and Archaea were purified and sequenced. The 16S rRNA gene sequences of the dominant archaeal bands found in the inoculum, and UASB sludge fed with raw sewage, CEPS pretreated wastewater, and synthetic sewage were closely associated with Methanosaeta concilii. In the UASB sludge fed with synthetic sewage, another dominant band associated with an uncultured archaeon 39-2 was found together with M. concilii.

Development of a naturally derived coagulant for water and wastewater treatment

Seeds of the pan tropical tree, Moringa oleifera Lam. (M. oleifera) contain water soluble, positively charged proteins that act as an effective coagulant (molecular weight 13 kDa and isoelectric pH 10-11). Treatment studies are reviewed using the extracts of the crushed seed kernel and of the presscake (solids residue remaining after oil extraction). M. oleifera seed was incorporated into a pilot scale treatment system as a primary coagulant to clarify river water of 400 NTU in the wet season. Over 90% turbidity removal was achieved in the sedimentation stage at a dosage of 100 mgL-1. The filtrate turbidity was maintained well below 5 NTU. Subsequent trials were conducted in an adjacent works operating at 60 m3h-1. Treatment performance was comparable to that of using aluminium sulphate. Inlet turbidities of 270-380 NTU were consistently reduced to below 4 NTU. A comprehensive study was undertaken to evaluate the potential of using M. oleifera coagulant within a contact flocculation filtration (CFF) pilot rig. A wide range of operating conditions was evaluated in order to establish the useful “working envelope” for this process combination (filtration rates 5-20 mh-1, initial turbidities 20-50 NTU). Principal results are presented. As a coagulant within chemically enhanced primary sedimentation (CEPS) of a mixed wastewater, M. oleifera dosed at 150 mgL-1 gave additional removals (compared to a plain sedimentation control) of 40% biochemical oxygen demand (BOD) and chemical oxygen demand (COD) and in excess of 80% for suspended solids (SS). Subsequent laboratory work coupled an upflow anaerobic sludge blanket reactor (UASB) to CEPS. M. oleifera coagulant in the CEPS pre-treatment unit beneficially increased the ratio of soluble COD to volatile SS by a factor of ten compared to plain sedimentation and by a factor of three when dosing ferric chloride. The UASB yielded more biogas and gave enhanced COD removal. Recent work on the extraction and purification of the active coagulant components is reviewed and the prospects for commercialisation are considered.

Technical Feasibility of the Treatment of Domestic Wastewater by a CEPS-UASB System

Raw domestic wastewater was treated continuously under laboratory conditions for 170 days by a chemically enhanced primary sedimentation (CEPS) followed by an upflow anaerobic sludge blanket (UASB) reactor. The CEPS was carried out with 70 mg FeCl3l−1 from day 1 to 82 and with 24 ml l−1 of the water extract of Moringa oleifera seeds from day 83 to 170. Compared to the natural primary sedimentation (NPS), the CEPS increased the ratio of soluble chemical oxygen demand to volatile suspended solids (CODs/VSS) of the supernatant by a factor 3 and 10 respectively. Although the FeCl3 increased the CODs/VSS ratio, it caused a decrease of the soluble content of the wastewater by a factor 1.4. This resulted in a low influent concentration supplied to the UASB reactor and consequently a low biogas production. However, the reactor achieved 54 % removal of total COD (CODt) at a hydraulic retention time (HRT) of 2 hours and a volumetric loading rate (Bv) of 1.4 g COD l−1 d−1. The Moringa oleifera seeds, besides the increase of CODs/VSS ratio, also allowed to increase the CODs content of the wastewater by a factor 2.2. As a consequence, the reactor produced a higher amount of biogas. The reactor achieved 71 % removal of CODt at a HRT of 2 hours and a Bv of 4 g COD l−1 d−1. The UASB reactor operated without the need to discharge regularly the biological excess of sludge produced. The implementation of the CEPS can decrease the volume needed by a conventional one step UASB reactor by a factor 0.4.