Effect of amended soil and hydraulic load on enhanced biological nitrogen removal in lab-scale SWIS

To characterize the effect of amended soil on nitrogen removal in subsurface wastewater infiltration system (SWIS), culture, grass carbon, and zeolite were mixed to produce microbial inoculums, and then the optimal microbial inoculums, nutrient substance, cinder, and original soil were mixed to produce the soils through bioaugmentation. Results indicate that the microbial inoculums (culture+50% grass carbon+50% zeolite) and the amended soil (12.5% microbial inoculums+25% nutrient substrate+12.5% cinder+50% original soil) have the optimal biogenic stimulating properties, and the adsorption capacity of the amended soil are 1.216 mg-Pg(-1) and 0.495 mg-Ng(-1). The laboratory soil column experiment indicates that the efficient mode of nitrogen removal in lab-scale SWIS is adsorption-nitrification-denitrification and the nitrification/denitrification can be enhanced by the application of the amended soil. On average, the SWIS filled with amended soil converts 85% of ammonia nitrogen (NH(4)(+)-N) to NO(x)(-)-N and removes 49.8-60.6% of total nitrogen (TN), while the system filled with original soil removes 80% of NH(4)(+)-N and 31.3-43.2% of TN at 4-8 cm day(-1). Two systems are overloads at 10 cm day(-1). It is concluded that the microbial activities and nitrogen removal efficiencies are improved in SWIS after bioaugmentation.     

Study of the aerobic biodegradation of coke wastewater in a two and three-step activated sludge process

A laboratory-scale biological plant composed of two aerobic reactors operating at 35 degrees C was used to study the biodegradation of coke wastewater. The main pollutants to be removed are organic matter, especially phenols, thiocyanate and ammonium nitrogen. The concentrations of the main pollutants in the wastewater during the study ranged between 922 and 1,980 mg COD/L, 133 and 293 mg phenol/L, 176 and 362 mg SCN/L and 123 and 296 mg NH(4)(+)-N/L. The biodegradation of these pollutants was studied employing different hydraulic residence times (HRT) and final effluent recycling ratios in order to minimize inhibition phenomena attributable to the high concentrations of pollutants. During the optimisation of the operating conditions, the removal of COD, phenols and thiocyanate was carried out in the first reactor and the nitrification of ammonium took place in the second. The best results were obtained when operating at an HRT of 98 h in the first reactor and 86 h in the second reactor, employing a recycling ratio of 2. The maximum removal efficiencies obtained were 90.7, 98.9, 98.6 and 99.9% for COD, phenols, thiocyanate and NH(4)(+)-N, respectively. In order to remove nitrate, an additional reactor was also implemented to carry out the denitrification process, adding methanol as an external carbon source. Very high removal efficiencies (up to 99.2%) were achieved.     

Study on nitrogen removal enhanced by shunt distributing wastewater in a constructed subsurface infiltration system under intermittent operation mode

Subsurface wastewater infiltration system is an efficient and economic technology in treating small scattered sewage. The removal rates are generally satisfactory in terms of COD, BOD(5), TP and SS removal; while nitrogen removal is deficient in most of the present operating SWIS due to the different requirements for the presence of oxygen for nitrification and denitrification processes. To study the enhanced nitrogen removal technologies, two pilot subsurface wastewater infiltration systems were constructed in a village in Shenyang, China. The filled matrix was a mixture of 5% activated sludge, 65% brown soil and 30% coal slag in volume ratio for both systems. Intermittent operation mode was applied in to supply sufficient oxygen to accomplish the nitrification; meanwhile sewage was supplemented as the carbon source to the lower part in to denitrify. The constructed subsurface wastewater infiltration systems worked successfully under wetting-drying ratio of 1:1 with hydraulic loading of 0.081 m(3)/(m(2)d) for over 4 months. Carbon source was supplemented with shunt ratio of 1:1 and shunt position at the depth of 0.5m. The experimental results showed that intermittent operation mode and carbon source supplementation could significantly enhance the nitrogen removal efficiency with little influence on COD and TP removal. The average removal efficiencies for NH(3)-N and TN were 87.7 ± 1.4 and 70.1 ± 1.0%, increased by 12.5 ± 1.0 and 8.6 ± 0.7%, respectively.     

Biological nitrogen and phenol removal from saline industrial wastewater by submerged fixed-film reactor

In this study a biological nitrogen removal process using a submerged fixed-film reactor was applied to treat industrial wastewater with phenol (1g/l), a high nitrogen concentration (0.4 g N/l) and high salinity (30 g/l). The process consisted of a pre-denitrification system with a down-flow-up-flow biofilter (two columns, each with an effective volume of 21 l) packed with clayey schists from recycled construction material. The efficiency of the system for reducing COD, phenol concentration and total nitrogen was tested under different running conditions such as influent flow (10, 12 and 15 l/d), air loading (6.8 and 13.6m(3)/m(2)h) and effluent recirculation (300%, 400%, and 600%). The system demonstrated a high capacity for reducing COD concentration (95.75+/-0.72%), independently of running conditions. The aerobic column eliminated most of the phenol in the influent. Nitrogen removal took place mainly in the anoxic column, and was conditioned by the air loading in the aerated column, owing to the dependence of nitrification on the supply of oxygen. However, this process was not able to achieve a nitrogen oxidation superior to 63%, in spite of a sufficient supply of oxygen and the diluting effect of high recirculation (600%) on the phenol concentration in the influent. In spite of the limitations observed in the process of nitrification, results for the removal of total nitrogen were as high as 83%, owing to a combination of different processes for nitrogen removal.     

Confirming Pseudomonas putida as a reliable bioassay for demonstrating biocompatibility enhancement by solar photo-oxidative processes of a biorecalcitrant effluent

The performance of a sequencing batch reactor (SBR) seeded with aerobic granular sludge was studied. The lab-scale SBR treating domestic wastewater operated at a volumetric loading rate (VLR) of 0.75-3.41 kg COD/(m(3)d). The granule stability was related to the organic loading, and high loading would be favorable for granule stability. Analysis of typical cycle showed that granular sludge had good ability to simultaneously remove nitrogen and phosphorus. Most organic substances were removed at the anaerobic stage. At the aerobic stage, simultaneous nitrification and denitrification (SND) happened with phosphorus absorption. The SBR had good removal performance for organic matter and phosphate. However, the total nitrogen (TN) removal performance was ordinary, with average removal efficiency of about 52%. Batch experiments indicated that increases of influent C/N ratio and a large percentage of granule in the sludge were conducive for SND in SBR.     

Studies of a membrane aerated bioreactor for wastewater treatment

This study was undertaken specifically to collect engineering and process performance information on the behaviour of a membrane supported bio-film in a well-characterised bioreactor. A novel membrane aerated bioreactor has been tested in the laboratory on synthetic sewage, and also piloted for the treatment of primary effluent at a local municipal wastewater treatment plant. The reactor's design employs gas-permeable, hollow-fibre membranes that are submerged in the wastewater. Air is blown through the inside of the fibres. Aeration costs for such a reactor are likely to be lower than conventional systems because gas compression is not required. Thick bio-films grow on the outside of the fibres and effectively treat the wastewater. The bio-films grown in this manner are capable of biological oxygen demand (BOD) removal, nitrification, denitrification, and simultaneous sludge digestion. The character of the bio-film is very different from conventional bio-films grown on inert surfaces, since in this case the bio-film receives its oxygen and the wastewater components from different directions. In pilot studies, a reactor having a hydraulic residence time of 6.5 h and equipped with no sedimentation tank achieved 70–75% BOD and nitrogen removal, and produced an effluent containing less than 30 mg/l total suspended solids.     

Simultaneous removal of phenol, ammonium and thiocyanate from coke wastewater by aerobic biodegradation

A laboratory-scale activated sludge plant composed of a 20 L volume aerobic reactor followed by a 12 L volume settling tank and operating at 35 degrees C was used to study the biodegradation of coke wastewater. The concentrations of ammonium nitrogen (NH(4)(+) -N), phenols, chemical oxygen demand (COD) and thiocyanate (SCN(-)) in the wastewater ranged between 504 and 2,340, 110 and 350, 807 and 3,275 and 185 and 370 mg/L, respectively. The study was undertaken with and without the addition of bicarbonate. The addition of this inorganic carbon source was necessary to favour nitrification, as the alkalinity of the wastewater was very low. Maximum removal efficiencies of 75%, 98% and 90% were obtained for COD, phenols and thyocianates, respectively, without the addition of bicarbonate. The concentration of ammonia increased in the effluent due to both the formation of NH(4)(+) as a result of SCN(-) biodegradation and to organic nitrogen oxidation. A maximum nitrification efficiency of 71% was achieved when bicarbonate was added, the removals of COD and phenols being almost similar to those obtained in the absence of nitrification. Batch experiments were performed to study the influence of pH and alkalinity on the biodegradation of phenols and thiocyanate.     

Characterization of stone powder sludge foams and their application to wastewater treatment: Role of pore connectivity

Ceramic foams with different relative densities (i.e., different extension ratios) were successfully prepared from stone powder sludge (SPS) via a foaming and gelcasting method. Ceramic foams with various relative densities were characterized in terms of porosity, specific surface area, and water absorption capacity. The porosity of the ceramic foams ranged from 35% to 78%, depending on the extension ratio. The specific surface area and water absorption capacity increased as the relative density decreased (i.e., increasing extension ratio), resulting in enhanced pore connectivity. The feasibility of the SPS foams as microorganism-immobilized carriers and the effect of inner pores on simultaneous nitrification/denitrification were tested through wastewater treatment experiments. The results from fixed-bed reactors packed by SPS carriers with different relative densities revealed that the organic removal efficiency for all reactors was greater than 90%. In addition, the nitrification/denitrification experimental results suggested that the increase of the NH₄⁺-N loading rate resulted in the improvement of nitrification efficiency and the decrease of nitrogen loss in all reactors. However, no significant correlation between relative density (i.e., extent of inner pore development) and nitrification/denitrification efficiency was found.     

Biological nitrate removal from wastewater of a metal-finishing industry

An upflow packed bed reactor at laboratory scale has been operated for a continuous period of 5 months to investigate the technical feasibility of biological nitrate removal applied to the effluent of the coagulation-sedimentation wastewater of a metal-finishing industry. The reactor was fed with industrial wastewater in a five-fold dilution to reproduce the global spill in the factory (20/80, industrial wastewater/domestic wastewater) with a concentration of nitrate between 141 and 210 gNO(3)-N/m(3). Methanol was added as a carbon source for denitrification. Inlet flow rate was progressively increased from 9 to 40 L/day (nitrogen input load from 45 to 250 gNO(3)-N/(m(3)h)). The highest observed denitrification rate was 135 gNO(3)-N/(m(3)h) at a nitrate load of 250 gNO(3)-N/(m(3)h), and removal efficiencies higher than 90% were obtained for loads up to 100 gNO(3)-N/(m(3)h). A mass relation between COD consumed and NO(3)-N removed around 3.31 was observed. Better results were achieved in a previous stage using tap water with nitrate added as a sole pollutant as a synthetic feed (critical load of 130 gNO(3)-N/(m(3)h) and denitrification rate of 200 gNO(3)-N/(m(3)h) at a nitrate load of 250 gNO(3)-N/(m(3)h)). This fact could indicate that the chemical composition of the industrial source hinders to some extent the performance of the biological process. Whatever case, results demonstrated the viability of the denitrification process for the global industrial wastewater. A simple model based on Monod kinetics for substrate consumption, and constant biomass concentration was applied to model the industrial wastewater treatment, and a reasonably good fitting was obtained.     

Study on treatment of coking wastewater by biofilm reactors combined with zero-valent iron process

Experiments were conducted to investigate the behavior of the integrated system with biofilm reactors and zero-valent iron (ZVI) process for coking wastewater treatment. Particular attention was paid to the performance of the integrated system for removal of organic and inorganic nitrogen compounds. Maximal removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH(3)-N) and total inorganic nitrogen (TIN) were up to 96.1, 99.2 and 92.3%, respectively. Moreover, it was found that some phenolic compounds were effectively removed. The refractory organic compounds were primarily removed in ZVI process of the integrated system. These compounds, with molecular weights either ranged 10,000-30,000 Da or 0-2000 Da, were mainly the humic acid (HA) and hydrophilic (HyI) compounds. Oxidation-reduction and coagulation were the main removal mechanisms in ZVI process, which could enhance the biodegradability of the system effluent. Furthermore, the integrated system showed a rapid recovery performance against the sudden loading shock and remained high efficiencies for pollutants removal. Overall, the integrated system was proved feasible for coking wastewater treatment in practical applications.     

Evaluation of potential integration of entrapped mixed microbial cell and membrane bioreactor processes for biological wastewater treatment/reuse

The challenge of biological wastewater treatment process is the design and operation of effective retention of mixed microbial cells within the reactor. Entrapped mixed microbial cell (EMMC) technology is designed to entrap the mixed microbial cells in polymeric carriers; membrane bioreactor (MBR) process utilizes membrane sheets/fibers to effectively retain the biomass in the reactor. These two biotechnologies are considered potential alternatives for conventional biological treatment/reuse because of their capability of retaining high concentration of biomass in the reactor, or in other words increasing the solid retention time (SRT). The simultaneous removal of organics and nitrogen were investigated using a modified EMMC system design. The modified EMMC system demonstrated higher organic and nitrogen removal performance due to high SRT. Compared to single-stage MBR process operated at similar conditions, the modified EMMC system was able to achieve slightly lower organic removal, comparable nitrification, and higher total nitrogen removal. One limitation in applying an EMMC only treatment process regime for potential reuse of treated wastewater is that such an operation requires the removal of pathogens and large particles if disinfection and solid/liquid separation were not followed. The major challenge of MBR process to overcome is membrane fouling, and the high energy consumption associated with fouling control. The intrinsic features of EMMC process including high SRT, low, and stabilized effluent suspended biomass concentration may significantly reduce the chance and extent of membrane fouling; while the membrane filtration can further polish the effluent quality from EMMC process. Therefore, integrating MBR and EMMC is strongly recommended because it may be a ??break-through?? for solving the membrane fouling problem and in improving effluent quality for potential reuse.     

Tertiary treatment of textile wastewater with combined media biological aerated filter (CMBAF) at different hydraulic loadings and dissolved oxygen concentrations

An up-flow biological aerated filter packed with two layers media was employed for tertiary treatment of textile wastewater secondary effluent. Under steady state conditions, good performance of the reactor was achieved and the average COD, NH(4)(+)-N and total nitrogen (TN) in the effluent were 31, 2 and 8mg/L, respectively. For a fixed dissolved oxygen (DO) concentration, an increase of hydraulic loading resulted in a decrease in substrate removal. With the increase of hydraulic loadings from 0.13 to 0.78m(3)/(m(2)h), the removal efficiencies of COD, NH(4)(+)-N and TN all decreased, which dropped from 52 to 38%, from 90 to 68% and from 45 to 33%, respectively. In addition, the results also confirmed that the increase of COD and NH(4)(+)-N removal efficiencies resulted from the increase of DO concentrations, but this variation trend was not observed for TN removal. With the increase of DO concentrations from 2.4 to 6.1mg/L, the removal efficiencies of COD and NH(4)(+)-N were 39-53% and 64-88%, whenas TN removal efficiencies increased from 39 to 42% and then dropped to 35%.     

Influence of hydraulic loading and air flowrate on urban wastewater nitrogen removal with a submerged fixed-film reactor

Nutrient disposal to sensitive areas, particularly nitrogen and phosphorus from wastewater treatment plants, provokes eutrophication reducing water quality. Fixed film technology is widely used for the removal of organic matter and nitrogen by the biological process of nitrification-denitrification. This paper studies a nitrification and post-denitrification lab-scale plant with a downflow aerobic submerged filter for removal of organic matter and nitrification, followed by an anoxic upflow biofilter for denitrification. Recycled construction material (clay shists) was employed as support material and methanol was used as carbon source. After 2 weeks of acclimation in which nitrification reached steady-state conditions, different hydraulic loadings (0.35-1.59 m(3)/m(2)h) and air flowrates (7.78-43.5 m(3)/m(2)h) were applied for 1 year. The highest hydraulic loading which complied with the EU regulation on nitrogen disposal was 0.71 m(3)/m(2)h (1.6 h). Hydraulic retention time (HRT), which corresponded to a nitrogen removal of 0.64 kg N/m(3) per day operating at an air flowrate of 25.6 m(3)/m(2)h. Concerning to organic matter removal efficiency, the aerobic reactor accepted a maximum chemical oxygen demand (COD) volumetric loading of 16.0 kg COD/m(3) per day with a 75% COD removal efficiency. For all the tests carried out, suspended solids (SS) concentration in the outlet water was less than 35 mg/l.     

Aerobic decolourization of the indigo dye-containing textile wastewater using continuous combined bioreactors

An aerobic bioprocess was applied to Indigo dye-containing textile wastewater treatment aiming at the colour elimination and biodegradation. A combined aerobic system using continuous stirred tank reactor (CSTR) and fixed film bioreactor (FFB) was continuously operated at constant temperature and fed with the textile wastewater (pH: 7.5 and total chemical oxygen demand (COD): 1185 mg l(-1)). The CSTR is a 1l continuous flow stirred tank reactor with a 700 ml working volume, and operated with a variable wastewater loading rate (WLR) from 0.92 to 3.7 g l(-1) d(-1). The FFB is a 1.5l continuous flow with three compartments packed with a rippled cylindrical polyethylene support, operated with a variable WLR between 0.09 and 0.73 g l(-1) d(-1). The combined two bioreactors were inoculated by an acclimated microbial consortium and continuously operated with four total WLR. This system presented high COD elimination and colour removal efficiencies of 97.5% and 97.3%, respectively, obtained with a total hydraulic retention time (HRT) of 4 days and total WLR of 0.29 g l(-1) d(-1). The effects of WLR on absorption phenomena on the yield of conversion of substrate on biomass (R(TSS/COD)) and on the yield of conversion of substrate on active biomass (R(VVS/COD)) are discussed. The increase of WLR and the decrease of HRT diminished the performances of this system in terms of decolourization and COD removal explained by the sloughing of biofilm, and the washout phenomena.     

Operational strategies for nitrogen removal in granular sequencing batch reactor

This study investigated the effects of different operational strategies for nitrogen removal by aerobic granules with mean granule sizes of 1.5mm and 0.7 mm in a sequencing batch reactor (SBR). With an alternating anoxic/oxic (AO) operation mode without control of dissolve oxygen (DO), the granular sludge with different size achieved the total inorganic nitrogen (TIN) removal efficiencies of 67.8-71.5%. While under the AO condition with DO controlled at 2mg/l at the oxic phase, the TIN removal efficiency was improved up to 75.0-80.4%. A novel operational strategy of alternating anoxic/oxic combined with the step-feeding mode was developed for nitrogen removal by aerobic granules. It was found that nitrogen removal efficiencies could be further improved to 93.0-95.9% with the novel strategy. Obviously, the alternating anoxic/oxic strategy combined with step-feeding is the optimal way for TIN removal by granular sludge, which is independent of granule size.     

Simultaneous nitrification-denitrification and phosphorus removal in a fixed bed sequencing batch reactor (FBSBR)

Biological nutrient removal (BNR) was investigated in a fixed bed sequencing batch reactor (FBSBR) in which instead of activated sludge polypropylene carriers were used. The FBSBR performance on carbon and nitrogen removal at different loading rates was significant. COD, TN, and phosphorus removal efficiencies were at range of 90-96%, 60-88%, and 76-90% respectively while these values at SBR reactor were 85-95%, 38-60%, and 20-79% respectively. These results show that the simultaneous nitrification-denitrification (SND) is significantly higher than conventional SBR reactor. The higher total phosphorus (TP) removal in FBSBR correlates with oxygen gradient in biofilm layer. The influence of fixed media on biomass production yield was assessed by monitoring the MLSS concentrations versus COD removal for both reactors and results revealed that the sludge production yield (Y(obs)) is significantly less in FBSBR reactors compared with SBR reactor. The FBSBR was more efficient in SND and phosphorus removal. Moreover, it produced less excess sludge but higher in nutrient content and stabilization ratio (less VSS/TSS ratio).     

Instability of biological nitrogen removal in a cokes wastewater treatment facility during summer

Failure in nitrogen removal of cokes wastewater occurs occasionally during summer season (38 degrees C) due to the instability of nitrification process. The objective of this study was to examine why the nitrification process is unstable especially in summer. Various parameters such as pH, temperature, nutrients and pollutants were examined in batch experiments using activated sludge and wastewater obtained from a full-scale cokes wastewater treatment facility. Batch experiments showed that nitrification rate of the activated sludge was faster in summer (38 degrees C) than in spring or autumn (29 degrees C) and the toxic effects of cyanide, phenol and thiocyanate on nitrification were reduced with increasing temperature. Meanwhile, experiment using continuous reactor showed that the reduction rate in nitrification efficiency was higher at 38 degrees C than at 29 degrees C. In conclusion, the instability of full-scale nitrification process in summer might be mainly due to washing out of nitrifiers by fast growth of competitive microorganisms at higher temperature under increased concentrations of phenol and thiocyanate.     

Application of Box-Wilson experimental design method for 2,4-dinitrotoluene treatment in a sequential anaerobic migrating blanket reactor (AMBR)/aerobic completely stirred tank reactor (CSTR) system

A sequential aerobic completely stirred tank reactor (CSTR) following the anaerobic migrating blanket reactor (AMBR) was used to treat a synthetic wastewater containing 2,4-dinitrotoluene (2,4-DNT). A Box-Wilson statistical experiment design was used to determine the effects of 2,4-DNT and the hydraulic retention times (HRTs) on 2,4-DNT and COD removal efficiencies in the AMBR reactor. The 2,4-DNT concentrations in the feed (0-280 mg/L) and the HRT (0.5-10 days) were considered as the independent variables while the 2,4-DNT and chemical oxygen demand (COD) removal efficiencies, total and methane gas productions, methane gas percentage, pH, total volatile fatty acid (TVFA) and total volatile fatty acid/bicarbonate alkalinity (TVFA/Bic.Alk.) ratio were considered as the objective functions in the Box-Wilson statistical experiment design in the AMBR. The predicted data for the parameters given above were determined from the response functions by regression analysis of the experimental data and exhibited excellent agreement with the experimental results. The optimum HRT which gave the maximum COD (97.00%) and 2,4-DNT removal (99.90%) efficiencies was between 5 and 10 days at influent 2,4-DNT concentrations 1-280 mg/L in the AMBR. The aerobic CSTR was used for removals of residual COD remaining from the AMBR, and for metabolites of 2,4-DNT. The maximum COD removal efficiency was 99% at an HRT of 1.89 days at a 2,4-DNT concentration of 239 mg/L in the aerobic CSTR. It was found that 280 mg/L 2,4-DNT transformed to 2,4-diaminotoluene (2,4-DAT) via 2-amino-4-nitrotoluene (2-A-4-NT) and 4-amino-2-nitrotoluene (4-A-2-NT) in the AMBR. The maximum 2,4-DAT removal was 82% at an HRT of 8.61 days in the aerobic CSTR. The maximum total COD and 2,4-DNT removal efficiencies were 99.00% and 99.99%, respectively, at an influent 2,4-DNT concentration of 239 mg/L and at 1.89 days of HRT in the sequential AMBR/CSTR.     

Biological removal of cyanide compounds from electroplating wastewater (EPWW) by sequencing batch reactor (SBR) system

Biological treatment system especially, sequencing batch reactor (SBR) system could not be applied to treat the raw electroplating wastewater (EPWW) due to the low organic matter concentration of 10+/-3 mg-BOD5/L and toxic of high cyanide concentration of 23.0+/-2.2 mg-CN/L. However, EPWW could be used as the nitrogen source for the bio-sludge of SBR system. And 10% of EPWW (the final cyanide concentration of 2.3+/-0.2 mg/L) was most suitable to supplement into the wastewater as the nitrogen source. SBR system showed the highest COD, BOD5, TKN and cyanide removal efficiencies of 79+/-2%, 85+/-3%, 49.0+/-2.1% and 97.7+/-0.7%, respectively with 4-times diluted Thai-rice noodle wastewater (TRNWW) containing 10% EPWW and 138 mg/L NH4Cl (BOD5: TN of 100:10) at SRT of 72+/-13 days (under organic and cyanide loadings of 0.40 kg-BOD5/m3 d and 0.0023 kg-CN/m3 d, respectively). However, the effluent ammonia was still high of 22.6+/-0.4 mg-N/L while the effluent nitrate and nitrite was only 9.9+/-0.4 and 1.2+/-0.9 mg-N/L, respectively. And SVI and effluent SS of the system were higher than 95 and 75 mg/L, respectively.     

Anaerobic treatment of olive mill wastewater and piggery effluents fermented with Candida tropicalis

Olive mill wastewater (OMW) contains high concentrations of phenolic compounds that are inhibitory to many microorganisms making it difficult to treat biologically prior to discharge in waterways. The total mono-cyclic phenol reduction in OMW in this study was carried out by aerobic pre-treatment using the yeast Candida tropicalis in a 18 L batch reactor at 30 degrees C for 12 days followed by anaerobic co-digestion. A COD removal of 62% and a reduction in the total mono-cyclic phenol content by 51% of the mixture was achieved in the aerobic pre-treatment. Pig slurry was added as co-substrate to supplement the low nitrogen levels in the olive mill wastewater. Subsequent anaerobic treatment was carried out in a 20L fixed-bed reactor at 37 degrees C and HRT between 11 and 45 days. After a long start-up period, the OLR was increased from 1.25 to 5 kg COD m(-3)day(-1) during the last 30 days, resulting in subsequent increase in overall COD removal and biogas production, up to maximum values of 85% and 29 L(biogas)L(reactor)(-1)day(-1), respectively. Methane content of the biogas produced from the anaerobic digestion ranged between 65% and 74%.