Moving bed biofilm reactor to treat wastewater

This review carries out a comparative study of advanced technologies to design, upgrade and rehabilitate wastewater treatment plants. The study analyzed the relevant researches in the last years about the moving bed biofilm reactor process with only attached biomass and with hybrid biomass, which combined attached and suspended growth; both could be coupled with a secondary settling tank or microfiltration/ultrafiltration membrane as a separation system. The physical process of membrane separation improved the organic matter and NH₄ ⁺-N removal efficiencies compared with the settling tank. In particular, the pure moving bed biofilm reactor–membrane bioreactor showed average chemical oxygen demand, biochemical oxygen demand on the fifth day and total nitrogen removal efficiencies of 88.32, 90.84 and 60.17%, respectively, and the hybrid moving bed biofilm reactor–membrane bioreactor had mean chemical oxygen demand, biochemical oxygen demand on the fifth day and total nitrogen reduction percentages of 91.18, 97.34 and 68.71%, respectively. Moreover, the hybrid moving bed biofilm reactor–membrane bioreactor showed the best efficiency regarding organic matter removal for low hydraulic retention times, so this system would enable the rehabilitation of activated sludge plants and membrane bioreactors that did not comply with legislation regarding organic matter removal. As the pure moving bed biofilm reactor–membrane bioreactor performed better than the hybrid moving bed biofilm reactor–membrane bioreactor concerning the total nitrogen removal under low hydraulic retention times, this system could be used to adapt wastewater treatment plants whose effluent was flowed into sensitive zones where total nitrogen concentration was restricted. This technology has been reliably used to upgrade overloaded existing conventional activated sludge plants, to treat wastewater coming from textile, petrochemical, pharmaceutical, paper mill or hospital effluents, to treat wastewater containing recalcitrant compounds efficiently, and to treat wastewater with high salinity and/or low and high temperatures.     

Effects of sludge pretreatment on sludge reduction in a lab-scale anaerobic/anoxic/oxic system treating domestic wastewater

Excess sludge disposal is one of the serious challenges in biological wastewater treatment. Reduction of sludge production would be an ideal way to solve sludge-associated problems rather than the post-treatment of the sludge produced. In this study, a new wastewater treatment process combining anaerobic/anoxic/oxic system with thermochemical sludge pretreatment was tested in a laboratory scale experiment. In this study, the effects of the sludge pretreatment on the excess sludge production in anaerobic/anoxic/oxic were investigated. The system was operated in two Runs (1 and 2). In Run 1, the system was operated as a reference and in Run 2, a part of the mixed liquid was pretreated thermochemically and was returned to the bioreactor. The average solubilization efficiency of pretreated sludge was found to be about 35 % during the study period of 220 days. Sludge production rate in Run 2 was less than that in Run 1 by about 52 %. Total phosphorous was removed by enhanced biological phosphorous removal with the removal efficiency of 83–87 % and 81–83 % for Run 1 and Run 2, respectively. Total nitrogen removal in Run 2 (79–82 %) was slightly higher than that in Run 1 (68–75 %). The mixed liquor suspended solids/mixed liquor volatile suspended solids ratio was identical after both runs in the range 78–83 %. The effluent water qualities were not significantly affected when operated with thermochemical pretreatment at pH 11 and 60 °C for 3 h during 7 months. From the present study it is concluded that thermochemical sludge pretreatment of anaerobic/anoxic/oxic process plays an important role in reduction of sludge production.     

Innovative anaerobic upflow sludge blanket filtration combined bioreactor for nitrogen removal from municipal wastewater

In this research, a novel laboratory scale anaerobic/upflow sludge blanket filtration combined bioreactor was designed and operated to improve the efficiency of the upflow sludge blanket filtration process for the simultaneous removal of phosphorus and nitrogen from wastewater. The anaerobic/upflow sludge blanket filtration technique was developed by adding an anaerobic reactor to its influent and operated by varying the main process parameters in order to gain the optimum conditions. The results showed that biological removal efficiency of nitrogen and preservation of sludge blanket strongly depend on wastewater characteristics, hydraulic retention time, sludge age and process controlling parameters. The combined bioreactor performed a total nitrogen removal efficiency of 96.6 % with the sludge age of 25 days, total hydraulic retention time of 24 h and optimum “chemical oxygen demand/nitrogen/phosphorus” ratio of 100/ 5/1. This ratio also improved the compaction quality of sludge blanket in the upflow sludge blanket filtration clarifier. The average specific nitrification and denitrification rates occurred during the process can be expressed as 4.43 mg NO_x-N produced/g VSS.d and 5.50 mg NOx-N removed/g VSS.d at the optimum ratio, respectively. To avoid sludge rising due to denitrification process, the optimum total hydraulic retention time of 16 to 24 h was achieved based on the effluent quality. This study suggested that the anaerobic/upflow sludge blanket filtration bioreactor at the optimum operational conditions can be an effective process for removal of nutrients from municipal wastewater.     

Activated sludge properties in a submerged membrane bioreactor: effect of COD/TKN ratio of wastewater

The properties of activated sludge are very important in a membrane bioreactor (MBR) in terms of membrane fouling. The most important parameters affecting the membrane fouling can be listed as mixed liquor suspended solid (MLSS) concentration, soluble microbial products (SMPs), extracellular polymeric substances (EPSs), floc size, aeration and viscosity of both supernatant and activated sludge. The COD/TKN ratio also affects the physical properties of sludge in MBR system. This study aimed to investigate the effect of chemical oxygen demand-to-total Kjeldahl nitrogen (COD/TKN) ratio of feed wastewater treated in an MBR on biological components of activated sludge. The activated sludge characteristics were determined by quantitative analyses such as MLSS, EPS, SMP, floc size distribution, zeta potential, relative hydrophobicity and capillary suction time in a submerged MBR treating simulated wastewater having different COD/TKN ratios (16, 56 and 107). The COD and TKN removal efficiencies were found to be almost equal in the sMBRs having different COD/TKN ratios. However, it was seen that the EPS content and SMP concentration in the supernatant increased with increasing COD/TKN ratio. The results indicated that the COD/TKN ratio of feed should be considered as an effective parameter on activated sludge properties in sMBR systems.     

Applicability of upflow anaerobic sludge blanket (UASB) reactor for typical sewage of a small community: its biomass reactivation after shutdown

In this study, the characteristics of sewage of small community were determined for 6 months to ascertain the type of treatment required in subtropical conditions. The results demarcated sewage of this community as a medium-strength wastewater (chemical oxygen demand: 475 mg/L, biochemical oxygen demand: 240 mg/L and total suspended solids: 434 mg/L). Chemical oxygen demand to sulphate ratio of the sewage (11.6) established that it was amenable to anaerobic digestion. The temperature, strength, biodegradability and components of sewage were suitable for anaerobic digestion, and thus, upflow anaerobic sludge blanket reactor (UASB) was selected for its treatment. These reactors are often shutdown in small communities due to environmental and/or socio-economic factors. The ability of two UASB reactors, seeded with cow dung (UASB_CD) and activated sludge of a dairy treatment plant (UASB_ASDIT) to restart after a long idle period of 12 months, was investigated along with sludge analysis by scanning electron microscope. Biomass in both reactors reactivated rapidly after shutdown period and within 30 days after substrate feeding achieved uniform removal efficiencies for chemical oxygen demand, total suspended solids, total dissolved solids, chloride and oil and grease. Chemical oxygen demand removal efficiency of both reactors became uniform and remained close to 80% after 30 days through reactivation of microbes in sludge bed due to adequate food and temperature conditions. During restart-up, at an average organic loading rate of 0.902 kg COD/m³ per day, methane yields of 0.091 and 0.084 m³/kg COD removed were achieved for UASB_CD and UASB_ASDIT reactors, respectively.     

Kinetic analysis of enhanced biological phosphorus removal in a hybrid integrated fixed film activated sludge process

Hybrid integrated fixed film activated sludge is a promising process for the enhancement of nitrification, denitrification and phosphorus removal in conventional activated sludge systems that can be used for upgrading biological nutrient removal, particularly when they have space limitations or need modifications that will require large monetary expenses. In this research, successful implementation of hybrid integrated fixed film activated sludge process at temperate zone wastewater treatment facilities has been studied by the placement of fixed film media into aerobic, anaerobic and anoxic zones. The primary objective of this study was to investigate the incorporation of enhanced biological phosphorus removal into hybrid integrated fixed film activated sludge systems and study the interactions between the fixed biomass and the mixed liquor suspended solids with respect to substrate competition and nutrient removal efficiencies. A pilot-scale anaerobic-anoxic-oxic configuration system was used. The system was operated at different mean cell residence times and influent chemical oxygen demand/total phosphorus ratios and with split influent flows. The experimental results confirmed that enhanced biological phosphorus removal could be incorporated successfully into hybrid integrated fixed film activated sludge system, but the redistribution of biomass resulting from the integration of fixed film media and the competition of organic substrate between enhanced biological phosphorus removal and denitrification would affect performances. Also, kinetic analysis of the reactor with regarding to phosphorus removal has been studied with different kinetic models and consequently the modified Stover-Kincannon kinetic model has been chosen for modeling studies and experimental data analysis of the hybrid integrated fixed film activated sludge reactor.     

Treatability of landfill leachate by combined upflow anaerobic sludge blanket reactor and aerated lagoon

Continuous upflow anaerobic sludge blanket reactor performs more favorably at the higher organic loading rate than other anaerobic treatment. The treatment of municipal landfill leachate of Shiraz??s city investigated using continuous flow anaerobic reactor and subsequently aerated lagoon. Landfill leachate has chemical oxygen demand of 45,000?C90,000?mg/L and ammonia nitrogen at 1,000?C2,500 and heavy metals that can impact biological treatments. Capacity of anaerobic and aerobic reactors is 10 and 20?L that operated at detention time of 2 and 4?days, respectively. Organic loading rate of upflow anaerobic sludge blanket is between 0.5?C20?g chemical oxygen demand/L/day. Chemical oxygen demand removal efficiencies are between 57?C87, 35?C70 and 66?C94% in the anaerobic, aerobic and whole system, respectively. As the entry, leachate organic loading rate increased from 1 to 20?g/L/day, the chemical oxygen demand removal efficiency reached a maximum of 71% and 84% in the anaerobic reactor and whole system, respectively, at high organic loading rate. Ammonium removal efficiency was about 54% after the aerobic stage.     

投加粉末活性炭对MBR运行性能的影响

试验研究比较了在相同的进水和运行条件下,反应器1(投加粉末活性炭,投加量为12.3 g,使其质量浓度达到1 100 mg/L)和反应器2(未投加粉末活性炭)的膜透水性及对污染物的去除效果,并分析了粉末活性炭可以提高膜过滤性能的相关机理。试验结果表明：反应器1的膜通量衰减速率明显小于反应器2;投加粉末活性炭改变了混合液的性质,也大大降低了混合液中胞外聚合物和微细胶体的含量,从而减缓了膜通量的下降速度,可以使系统长时间地以相对高的膜通量运行。     

Enhanced bioremoval of refractory compounds from dyeing wastewater using optimized sequential anaerobic/aerobic process

The upflow anaerobic sludge blanket process followed by the biological aerated filter process was employed to improve the removal of color and recalcitrant compounds from real dyeing wastewater. The highest removal efficiency for color was observed in the anaerobic process, at 8-h hydraulic retention time, seeded with the sludge granule. In the subsequent aerobic process packed with the microbe-immobilized polyethylene glycol media, the removal efficiency for chemical oxygen demand increased significantly to 75 %, regardless of the empty bed contact time. The average influent non-biodegradable soluble chemical oxygen demand was 517 mg/L, and the average concentration in effluent from the anaerobic reactor was 363 mg/L, suggesting the removal of some recalcitrant matters together with the degradable ones. The average non-biodegradable soluble chemical oxygen demand in effluent from the aerobic reactor was 87, 93, and 118 mg/L, with the removal efficiency of 76, 74, and 67 %, at 24-, 12-, and 8-h empty bed contact time, respectively. The combined anaerobic sludge blanket and aerobic cell-entrapped process was effective to remove the refractory compounds from real dyeing wastewater as well as in reducing organic loading to meet the effluent discharge limits. This integrated process is considered an effective and economical treatment technology for dyeing wastewater.     

Treatment of poultry slaughterhouse wastewater in upflow anaerobic filter under low upflow velocity

The wastewater discharged by poultry slaughterhouse industries are characterized mainly by high biochemical oxygen demand, high suspended solids and complex mixture of fats, proteins and fibers requiring systematic treatment prior to disposal. In this study, the performance of an upflow anaerobic filter reactor for treating Indian poultry slaughterhouse wastewater under low upflow velocity of 1.38 m/day at mesophilic temperature (29-35 °C) was investigated. The reactor was inoculated with anaerobic non-granular sludge from an anaerobic reactor treating the poultry slaughterhouse wastewater. The reactor took 147 days for complete start-up with removal efficiencies of total chemical oxygen demand and soluble chemical oxygen demand of 70 and 79 % respectively. The maximum total chemical oxygen demand removal efficiency of 78 % was achieved at an organic loading rate of 10.05 kg/m³/day and at an hydraulic retention time of 12 h. The average methane content varied between 46 and 56 % and methane yield at maximum removal efficiency was 0.24 m³ CH₄/kg COD_removed·day. Sludge granules of 1–2 mm were observed in between the packing media. Scanning electron microscope analysis revealed that sludge granules are composed of clumps of Methanosarcina clustered with less intertwined Methanosaeta fibre of granules. The lower velocity used in this study has achieved better performance of the reactor by creating active microbial formation with stable pH upto an organic loading rate of 14.3 kg/m³/day. This has proved that the poultry slaughterhouse wastewater can be treated using anaerobic filter reactor under low upflow velocity.     

Treatment of oily sludge by advanced oxidation process

For treating oily sludge, wet peroxide oxidation (WPO) and catalytic wet oxidation (CWO) were investigated. The CWO experiment was carried out in a 0.5?L batch reactor using FeCl₃ as catalyst. By using WPO, the effects of reaction parameters such as residence time, temperature, H₂O₂ excess, and initial COD were investigated. The results demonstrated that >80?% chemical oxygen demand (COD) was removed by CWO and >90?% COD was removed by WPO. Significantly, more of COD could be removed from the oily sludge by adding H₂O₂ in small doses. In conclusion, WPO was much more effective in the removal of organic compounds from oily sludge.     

Analysis of simulation tools and optimization of the operational conditions for biofilm activated sludge industrial process

Two simulation tools for industrial wastewater treatment plan using a biofilm activated sludge process are contrasted in this paper; biofilm activated sludge consists of moving bed biofilm reactor followed by conventional activated sludge. Wastewater treatment simulator software (BioWin) and general chemical engineering process software with an integrated mathematical model (Aspen Custom Modeler) are used as simulation tools under different configurations of wastewater treatment plant and influent conditions. The simulated results using BioWin do not fit the experimental concentrations of chemical oxygen demand and total suspension solids from a full-scale biofilm activated sludge process for cellulose and viscose plant wastewaters; however, using Aspen Custom Modeler the simulated results fit adequately. The differences between the two software are explained by the consideration or not of the predation and hydrolysis phenomena in the mathematical model. The trade-off between the models complexity and the goodness in the adjustment of the results shows that in the biofilm activated sludge process the inclusion of predation in the mathematical model becomes essential. The optimization tool included in Aspen Custom Modeler is selected to optimize the operational conditions using three objective functions: minimization of nutrients in the effluent, minimization of sludge production and maximization of excess sludge concentration. Operating cost of the process can be reduced up to 10% considering the optimal conditions proposed by the optimization tool.     

Study of syntrophic anaerobic digestion of volatile fatty acids using enriched cultures at mesophilic conditions

Volatile fatty acids are the most important intermediates in anaerobic digestion, and their degradations are extremely complicated thermodynamically. In this research, syntrophic anaerobic digestion of volatile fatty acids using enriched acetogenic and methanogenic cultures in a batch reactor at mesophilic conditions was investigated. Interactive effects of key microbiological and operating variables (propionic, butyric and acetic acids, retention time and methanogen to acetogen populations ratio) on the anaerobic degradation of volatile fatty acids were analyzed. Acetogenic and methanogenic anaerobes in the granular sludge from an up-flow anaerobic sludge blanket reactor were enriched at mesophilic conditions within a period of four weeks, separately. Enriched cultures were mixed with known proportions and then used in the bioreactor. Experiments were carried out based on central composite design and analyzed using response surface methodology. Four parameters (final concentrations of propionic, butyric and acetic acids and biogas production) were directly measured as response. Also, the optimum conditions for volatile fatty acid degradation were found to be 937.5 mg/L, 3275.5 mg/L, 2319.5 mg/L, 45 h and 2.2 proportions for propionic acid, butyric acid, acetic acid, retention time and methanogen to acetogen populations ratio, respectively (corresponding to maximum volatile fatty acid removal efficiencies and biogas production). The results of the verification experiment and the predicted values from the fitted correlations at the optimum conditions were in close agreement at a 95% confidence interval. The present study provides valuable information about the interrelations of quality and process parameters at different values of microbiological and operating variables.     

Nutrient removal from activated sludge mixed liquor by wastewater protozoa in a laboratory scale batch reactor

The aim of the study was to investigate the nutrient removal rate of three wastewater protozoan isolates. The study was carried out in a laboratory-scale batch reactor for a period of 120 h. in a four batch study. Aliquot samples were withdrawn from the reactor every 24 h. for the analysis of phosphate, nitrate, nitrite, ammonia, chemical oxygen demand, dissolved oxygen and pH, using standard methods. The results obtained in the different batches among the three isolates showed PO₄ ^2? removal rate ranging from 0.04 to 0.52 mg-PO₄ ^2?/L/h. while NO₃ ^? nitrate removal rates ranged from 0.08 to 0.16 mg-NO₃ ^?/L /h. Also NO₂- and NH₃ rates were observed to range between 0.022 and 0.087 mg-NO₂ ^?/L /h. 0.05 and 0.16 mg-NH₃ ^?/L /h, respectively. For the physicochemical parameters, there was no observed COD decrease; rather there was an increase and this was irrespective of isolates and experimental batches. However, dissolved oxygen concentration decreased drastically (below 1 mg/L) at the end of each batch while pH show a decrease after an initial 24 h. period and thereafter increased. This trend was also irrespective of isolates and experimental batches. Overall, the study has been able to show the effect of the test isolates on nutrient removal rates and other physicochemical parameters (COD, DO and pH) in activated sludge mixed liquor.     

Preservation of thermophilic mixed culture for anaerobic palm oil mill effluent treatment by convective drying methods

The generation of huge amount of liquid waste known as palm oil mill effluent is a major problem in oil palm industry. Meanwhile, anaerobic biodegradation of such organic effluent at thermophilic condition is a promising treatment technology due to its high efficiency. However, storage and transportation of thermophilic mixed culture sludge are challenging due to constant biogas generation and heating requirement. Hence, drying of thermophilic sludge was conducted to obtain dormant thermophiles and thus enables easier handling. In this study, thermophilic sludge was dried using heat pump at 22 and 32 °C as well as hot air oven at 40, 50, 60, and 70 °C. Subsequently, quality of dried sludge was examined based on most probable number enumeration, chemical oxygen demand, and methane yield. Average drying rate was found to increase from 3.21 to 17.84 g H₂O/m² min as drying temperatures increases while average moisture diffusivity values ranges from 5.07 × 10^?9 to 4.34 × 10^?8 m²/s. Oven drying of thermophilic mixed culture resulted in highest chemical oxygen demand removal and lowest log reduction of anaerobes at 53.41% and 2.16, respectively, while heat pump drying resulted in the highest methane yield and lowest log reduction of methanogens at 53.4 ml CH₄/g COD and 2.09, respectively. To conclude, heat pump at 22 °C was most suitable drying technique for thermophilic mixed culture as the original methane-producing capability was largely retained after drying, at a slightly lower yet still comparable chemical oxygen demand removal when palm oil mill effluent was treated with the rehydrated culture.     

Chemical oxygen demand fractions of municipal wastewater for modeling of wastewater treatment

When a new wastewater treatment plant is being designed by computer simulation, detailed data about organic fractions of influent wastewater (measured as chemical oxygen demand) are usually not available, but knowledge of the typical ranges of these fractions is indispensable. The influent chemical oxygen demand fractions can substantially influence the results of simulation-based design such as reactor volumes, solids residence time, effluent quality, oxygen demand, sludge production, etc. This article attempts to give an overview of wastewater organic fractions as modeling parameters and presents new chemical oxygen demand fractionation results from Hungary. According to the data from literature, the ratio of chemical oxygen demand components in raw wastewater is very different and the average composition is as follows: Inert particulate =17.1 %, slowly biodegradable = 57.9 %, inert soluble = 7.8 % and readily biodegradable = 17.5 %. The Hungarian wastewater samples were analyzed according to STOWA (Dutch foundation for applied water research) protocol and the obtained results were not much different from those of literature ( inert particulate = 23.7 %, slowly biodegradable = 49.8 %, inert soluble = 4.6 % and readily biodegradable = 21.9 %), but some typical characteristics were observed.     

Effect of the injection of pure oxygen into a membrane bioreactor on the elimination of bisphenol A

The effect of the injection of pure oxygen instead of air in a membrane bioreactor for the elimination of bisphenol A is investigated. A dynamic experiment was developed in a pilot plant where the aerobic reactor was continuously spiked with 1 mg L^?1 of bisphenol A. Air was injected for 10 days and then pure oxygen was injected for another 10 days. The bisphenol A concentration was determined in aqueous phases and activated sludge using simple and sensitive analytical methods based on different extraction procedures and liquid chromatography tandem mass spectrometry analysis. Enzymatic activity was also determined and toxicity tests were performed to discard that the spiked bisphenol A concentration could negatively affect the microorganisms in the bioreactor and, thus, the membrane bioreactor performance. The effluent bisphenol A concentration increased up to 0.26 mg L^?1 after 4 days in the air injection treatment, and up to 0.48 mg L^?1 after only 12 h in the oxygen injection treatment. In both cases, this was followed by a decrease in concentration despite the continuous spiking of bisphenol A into the bioreactor. In presence of pure oxygen, bisphenol A concentration reached background levels (below the limit of quantification) after 5 days. In contrast, when using air a total of 10 days were required to reach background levels. The injection of pure oxygen instead of air is an important innovation in wastewater treatment, allowing permanent elimination of organic contaminants, avoiding their return to the environment and ensuring the safety of water.     

The role of biodegradable particulate and colloidal organic compounds in biological nutrient removal activated sludge systems

The efficiency of denitrification and enhanced biological phosphorus removal in biological nutrient removal activated sludge systems is strongly dependent on the availability of appropriate carbon sources. Due to high costs of commercial compounds (such as methanol, ethanol, acetic acid, etc.) and acclimation periods (usually) required, the effective use of internal substrates is preferred. The aim of this study was to determine the effects of slowly biodegradable compounds (particulate and colloidal), as internal carbon sources, on denitrification, phosphate release/uptake and oxygen utilization for a full-scale process mixed liquor from two large wastewater treatment plants located in northern Poland. Since it is difficult to distinguish the effect of slowly biodegradable substrate in a direct way, a novel procedure was developed and implemented. Four types of one- and two-phase laboratory batch experiments were carried out in two parallel reactors with the settled wastewater without pre-treatment (reactor 1) and pre-treated with coagulation–flocculation (reactor 2). The removal of colloidal and particulate fractions resulted in the reduced process rates (except for phosphate release). The average reductions ranged from 13 % for the oxygen utilization rate during the second phase of a two-phase experiment (anaerobic/aerobic), up to 35 % for the nitrate utilization rate (NUR) during the second phase of a conventional NUR measurement.     

Anaerobic co-digestion of sewage and brewery sludge for biogas production and land application

In Thailand, sewage sludge production from the Bangkok metropolitan area can reach up to 63,000 ton/y by 2010. The Beer-Thai Company, Thailand, produces beer and generates lots of sludge as waste. Sewage sludge and brewery sludge can be used to generate energy which could be saved on the fossil fuels conventionally used as a source of energy. The possibility was explored to mix brewery sludge with sewage sludge at different mixing ratios for anaerobic digestion so that the energy can be generated as biogas and at the same time, digested sewage sludge can be used as fertilizer for agricultural applications. A batch anaerobic reactor under mesophilic condition for a digestion period of 40 days was used in the laboratory. The acrylic reactor was cylindrical with a working weight of 12 kg. The diameter was 23.7 cm and the height was 34.5 cm. Sludge mixtures at different ratios were fed into the reactors and the optimum mixing ratio was determined. Experimental results showed that the sludge mixture at ratio of 25:75 % by weight (sewage:brewery) yielded higher biogas production. A reduction in heavy metals and pathogens was observed at this ratio after the digestion indicating its safe use as fertilizer. Nitrogen content was about 4.95 % which is well above the commercial fertilizers. At optimum mixing ratio of 25:75, the amount of the generated biogas is 1.15×10⁶ m³/y. This large amount of biogas is equivalent to 1.44 million kWh/y of electricity, 561,000 L/y of diesel oil and 936,000 L/y of vehicle gasoline.     

Effects of aeration rate and detention time on thermophilic aerobic digestion of mixed sludge and its dewaterability

Thermophilic aerobic digestion (TAD) is a possible alternative for rapid sludge degradation and producing Class A biosolids. Aeration rate and detention time are two of the important parameters in TAD processes due to rapidly growing thermophilic bacteria population, limited solubility of oxygen at high temperatures and the need to prevent cooling of TAD process. Also the current knowledge and understanding of dewatering TAD biosolids are limited and incompelet. The objectives of this study were to investigate the effects of various aeration rates and detention time on some characteristics of mixed sewage sludge. Four glass-cylindrical digesters with 7 liters of sludge in each, placed in a water bath and were operated at 55 °C with the aeration rates of 2.14, 3.00,3.86 and 4.71 volume of air per volume of sludge per hours. It was found the increase in aeration rate decreased the required detention time in order to meet the Class A sludge regulations to reduce vector attraction. The values of oxidation-reduction potential (ORP) were changed from negative values to positiv ones and the values of pH were increased from around neutral to slightly basic, but this increases were occured at different detention times. TAD affected dewaterability of mixed sewage sludge and produced biosolids with higher specific resistance to filtration (lower dewaterability) than undigested sludge. Thermophically digested sludge has a good setteling behavior and air drying on sand bed.