ANAEROBIC TREATMENT AS A SUSTAINABLE TREATMENT OPTION FOR DOMESTIC WASTEWATER AND ORGANIC FRACTION OF MUNICIPAL SOLID WASTE Prof. Dr. Izzet Ozturk Istanbul Technical University, Environmental Engineering Faculty, 34469, Maslak, Istanbul/TURKEY
High-rate anaerobic treatment has been used widely for the treatment of many industrial and municipal wastewaters in the last decades Table 1. The benefits and drawbacks of anaerobic treatment of domestic sewage in the high-rate anaerobic systems
Under suitable conditions in an anaerobic sewage treatment system a bacterial population will develop that is compatible with the applied hydraulic and organic loads. Among the factors that determine the removal efficiency of biodegradable organic matter, the following are important: 1) The nature of the anaerobic matter to be removed 2) The suitability of environmental factors for anaerobic digestion 3) The retained amount of viable bacterial matter 4) The intensity of contact between the influent organic matter and the bacterial populations 5) The design of the anaerobic reactor 6) The retention time of sewage
Important environmental factors affecting anaerobic sewage digestion are: - temperature - pH - the presence of essential nutrients - the absence of excessive concentrations of toxic compounds • Nutrients (both macronutrients, nitrogen and phosphorus, and micronutrients) are abundantly available in sewage • Compounds that could exert a distinct toxic influence on the bacterial population as well as sulphide are generally absent in sewage • Besides type of the sewerage system (combined or seperate) also affects the composition of sewage as well as anaerobic treatment efficiency
High-rate anaerobic systems are generally applied in the temperature range of 25-40C • However, many recent researches conducted at all temperature conditions revealed that temperature is not a limiting factor in anaerobic treatment applications if the appropriate process design is chosen • When they are operated in lower temperature ranges (5-20C), various adaptations of the conventional high-rate reactor design are needed • The methanogenic biomass and the wastewater should be in sufficient contact (can be achieved by increased liquid upflow velocities)
Psychrophilic anaerobic treatment is an attractive option for wastewaters which are discharged at moderate to low temperatures (optimal temperature for psychrophilic microorganisms is around 17C) • Since domestic sewage has a temperature lower than 35C, heating is required during the mesophilic anaerobic treatment. Thus, anaerobic treatment systems allow substantially lower treatment costs due to their ability to operate at low temperatures (10-20C) • In recent years anaerobic treatment of wastewaters having low COD concentrations could be efficient especially with high-rate reactors such as UASB and fluidized bed reactors • Since low COD concentration of the influent results in very low substrate levels, low biogas productions will occur inside the reactor as well (HRT determines the reactor volume)
The most appropriate anaerobic system to treat domestic wastewater has been considered as the UASB reactor because of its simplicity, low investment and operation costs • Particulate organics are physically removed due to settling, adsorption and entrapment which is the first step in the anaerobic treatment and conversion of domestic sewage • The rate-limiting step in the overall digestion process is the hydrolysis of retained particulates which need relatively long retention times, depending on the applied process temperature
Direct anaerobic treatment of domestic sewage is generally not applied because of the fact that the high SS concentration in sewage causes considerable difficulties • Under low temperature conditions, the SS are hydrolysed very slowly and tend to acccumulate in the reactor (deteriorate the granular sludge) • In order to guarantee an efficient treatment of domestic sewage under low temperature conditions, at least part of the SS present in the wastewater should be removed before feeding the wastewater to a sludge bed reactor • On the other hand, it was reported that two stage systems are more suitable for anaerobic sewage treatment at low temperatures whereas at high temperatures single stage systems should be chosen • At two stage reactor approach generally long HRT’s are applied for SS hydrolysis at the first stage whereas short HRT’s are enough for methane production at the second stage
Specific biogas production rate is relatively low under psychrophilic conditions: - At low temperatures, the increase of CO2 dissolution in water might cause a decrease in the pH of the reactor because the solubility of gaseous compounds present in biogas increases with decrease in temperature - Low biodegradable organic matter concentration in the influent • Pilot and full-scale UASB reactor applications for domestic sewage are given in Table 2
Table 2. Pilot and full scale UASB reactor applications for domestic sewage”
On-site Anaerobic Treatment • Wastewaters are usually transported to centralised treatment plants through extended sewage networks however, decentralised wastewater treatment, i.e. community – or house- on-site treatment, may be more sustainable in some cases • Anaerobic on-site treatment is considered sustainable with its simple, thus cost-effective reactor design, small space requirement, low sludge production, low energy and nutrient demand, potential for energy production, high loading capacity, efficient removal of organic matter, possibility for nutrient recycling and suitability for small houses • The produced biogas is collected and utilised as renewable energy • Simple and easy-to-use anaerobic processes suitable for on-site treatment are septic tank, UASB-septic tank, and accumulation system
Figure 1.Flow scheme for a decentralised integrated system. 1. Pre-sedimentation tank, 2. UASB reactor, 3. RBC = Rotating Biological Contact Reactor, 4. UV/O3 = ultraviolet-ozone generator
Post-Treatment Alternatives • Anaerobic treatment is effective in removing biodegradable organic compounds, leaving mineralized compounds like ammonium, phosphate and sulfur in the solution. • These compounds therefore have to be removed by an additional post-treatment step to meet sufficiently the criteria for a sustainable environmental protection. • Besides it was reported that no pathogen removal could be achieved at low temperatures. Thus, anaerobic treatment of low strength wastewaters by UASB reactor should be considered as a pre-treatment alternative
Figure 3. Sectional view of a UASB + duckweed ponds + fish pond system
Biogas Stabilization Pond To land UASB (a) Sludge to drying beds Biogas Facultative Aerated Lagoon To river or land UASB (b) Sludge to drying beds Biogas Three Chamber Oxidation Pond To land UASB (c) Sludge to drying beds Figure 4. Treatment alternatives following UASB reactor (a) Stabilization pond (b) Facultative aerated lagoon (c) Oxidation pond
Anaerobic Digestion of OFMSW • Anaerobic digestion for the treatment of OFMSW was devoloped in the 1980’s and early 1990’s. • The biomethanization of OFMSW will become a very feasible option by applying subsidies to electricity production from wastes. • In most of the EU member countries the electricity generated from the reneweable sources is subsided with an additional Є0,1/kWh • Since 1990, more than 120 waste treatment facilities have been constructed in Europe. • In most of these plants, the anaerobic digestion is followed by an aerobic phase, for the additional pathogen removal, so that not only biogas but also compost is produced. • The nutrient rich supernatant from these digesters can be treated with MAP and struvite is produced which has a marketing value as a fertilizer (Also the supernetant from digesters has a potential of agricultural use).
Waste Characteristics • The organic fraction of municipal solid waste is rather a heterogenous substrate and the biogas yield in anaerobic treatment of OFMSW is dependent not only the the proces configuration, but also on the waste characteristics. • The waste characteristics is highly dependent on the collection system. • Source sorting of MSW generaly provides OFMSW of higher quality in terms of biogas yield and smaller quantities of non-biodegradable contaminants like plastics. • Mechanically seperated OFMSW which has a lower biogas potential is more contaminated, which leads to persistent handling problems and lower acceptability of the effluent product of the treatment process as fertilizer on agricultural land.
Waste Characteristics and Biogas Potential B0 : Maximum Methane Potential, G0 : Maximum Biogas Potential (%55 CH4)
Co-digestion Approach • The co-digestion concept involves the treatment of several waste types in a single treatment facility. • The profit of co-digestion in the anaerobic degredation proces is mainly within the folloving areas: • Increasing the methane yield • Improving the process stability • Achieving a better management of waste • The economical benefits of the collection and treatment of different types of wastes in a single treatment facility (Figure 5) • The fermentation products of OFMSW which is mainly VFA can be used as external carbon source for biological nutrient removal plants which suffers from organic carbon deficiency (Figure 5)
Qgas=0.1-0.3 m3/kg CODremoved (80% CH4) Biogas I Solar Energy Assisted Heating (if required!) Aerated or SBR Lagoon UASB Reactor Pump Domestic Wastewater Agricultural Use Grit Chamber Screen Qgas=0.30 m3/kg VSfed (65-70% CH4) Liquid Fermentation Products (High VFA) Biogas II Pulper Sludge Dewatering Plastics & Paper OFMSW ~ 8 to 10% TS Size Reduction Anaerobic Hdrolysis & Acid Fermentation Methane Reactor Heavy Fractions Seperate Collected Organic Fraction of Municipal Solid Waste (SC-OFMSW) Cake to Post Aerobic Composting Filtrate Liquid Fertilizer A general flow scheme for the implementation of co-digestion aproach and anaerobic treatment of municipal wastewater in a single treatment facility
Makro and micro nutrients C:N ratio pH Inhibitors/toxic compounds Biodegradable organic matter Dry matter Waste A Waste B Co-digestion Approach • The application of anaerobic co-digestion process may face problems due to some substrate characteristics. • High total solids content of typically 30 – 50% • High C:N ratio • Deficiecy in macro- and micronutrients • Content of toxic compounds (heavy metals, phthalates) • The key for codigestion lies in balancing several parametres in the co-substrate mixture.
Co-digestion with Sewage Sludge • The codigestion can be applied at existing facilities without great investments and it combines the treatment of the two largest municipal waste streams. • The addition of high solids concentration of OFMSW digester operated with sludge having a low TS concentration will be possible even in rather high concentrations • The stabilizing effect of sludge on the digestion of OFMSW has been confirmed with sludge doses between 8 – 20% of feedstock volatile solids (Kayhanian and Rich, 1996; Rivard et al., 1990). • For the codigestion of OFMSW with sewage sludge, the optimum C:N ratio of the feedstock was found to be in the range of 25 to 30 based on biodegradable carbon (Kayhanian and Tchobanoglolous, 1992; Kayhanian and Rich, 1996).
Co-digestion with Other Waste Types • Other organic waste types such as; • Livestock waste (manure) • Olive mill effluents • Macroalgae • Wastes generated by agro industries such as slaughterhouses, meat-processing industries can also be used as a co-substrate with OFMSW.