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Waste activated sludge production and management plays an important role in wastewater treatment plants (WWTPs), especially from an economic point of view. One possible approach is the anaerobic co-digestion of waste activated sludge with others organic substrates in mesophilic and thermophilic conditions in order to exploit the spare volume of existing reactors, recover energy from biogas production, and obtain a fertilizer as final product. The anaerobic trials were carried out at pilot scale, applying two organic loading rates (2.8 and 4.5 kg chemical oxygen demand (COD)/(m3.d)) with a hydraulic retention time of 16 and 21 days. Among agro-wastes, wine lees were chosen because of their continuous availability throughout the year, and their high COD content (up to 200-300 g/l, 70% soluble, on average). The addition of wine lees to activated sludge determined a higher biogas production (best yield was 0.40 Nm3/kgCODfed) improving the energetic balance of the sludge line of the WWTP. The characterization of both substrates fed and digester effluents was carried out in terms of heavy metals; comparison with EC proposed limits showed that, due to high content of Cu in wine lees, the loading rate of this agro-waste should be limited to maintain good characteristics of final biosolids. [ABSTRACT FROM AUTHOR]

This paper deals with an economic comparison between costs and incomes in small wastewater treatment plants where the anaerobic co-digestion process of sludge and biowaste with energy recovery is operated. Plants in the size range 1,000-30,000 persons equivalent (pe) were considered in the study: typical costs, comprehensive of capital and operating costs, were in the range €69-105 per person per year depending on the plant size: the smaller the size the higher the specific cost. The incomes deriving from taxes and fees for wastewater treatment are generally in the range €36-54 per person per year and can only partially cover costs in small wastewater treatment plants. However, the co-treatment of biowaste and the use of produced energy for extra credits (green certificates) determine a clear improvement in the possible revenues from the plant. These were calculated to be €23-25 per person per year; as a consequence the costs and incomes can be considered comparable for wastewater treatment plants (WWTPs) with size larger than 10,000 pe. Therefore, anaerobic co-digestion of biowaste and sludge can also be considered a sustainable solution for small wastewater treatment plants in rural areas where several different kinds of biowaste are available to enhance biogas production in anaerobic reactors. [ABSTRACT FROM AUTHOR]

The paper presents the results of two full-scale applications of the anaerobic co-digestion process of waste activated sludge together with the organic fraction of municipal solid wastes. The experiences were carried out at Viareggio and Treviso wastewater treatment plants (Italy). In the first plant, 3 tons per day of source sorted OFMSW were co-digested with waste activated sludge, increasing the organic loading rate from 1.0 to 1.2 kgTVS/m3d. This determined a 50O/o increase in biogas production. At Treviso WWTP, which has been working for 2 years, some 10 tons per day of separately collected OFMSW are treated using a low-energy consumption sorting line, which allows the removal of 99% and 90% of metals and plastics respectively. In these conditions, the biogas yield increased from 3,500 up to 17,500 m³/month. Industrial costs were evaluated less than 50 € per ton of organic waste, while the payback time was calculated as two years. [ABSTRACT FROM AUTHOR]

This paper presents a comparison of dry anaerobic digestion reactors fed with differently sorted municipal organic solid wastes. One reactor was fed with source sorted organic wastes and a second reactor was fed with mixed organic wastes consisting of grey wastes, mechanically selected municipal solid wastes and sludge. The two reactors utilised the same process (Valorga) and operational conditions at full scale. The results of the study emphasise the influence of the kind of treated material on the process performances, especially in terms of biogas and methane production, thus, energy reclamation. The reactor treating the source sorted organic waste and the reactor treating the mixed organic wastes generated some 200 m³ and 60 m³ of biogas per ton of waste treated, respectively, while the specific methane production was some 0.40 and 0.13 m³CH4/kgTVS, respectively. The mass balance and the final fate of the digested material from the two reactors were also clearly different. As for the costs, these were some 29 € per ton of treated waste (50% for personnel) and 53 €/ton for disposing of the rejected materials. Incomes were some 100 €/ton (on average) and an other 15 €/ton came from green certificates. The initial investment was 16 million Euros. [ABSTRACT FROM AUTHOR]

The results of the start-up of two digesters in mesophilic and thermophilic conditions, together with its steady results at several organic loading rates are described. A kinetic study is also carried out which allows one to estimate the ultimate methane production, together with the first-order kinetic constant. Operation at thermophilic temperature yields better results as it allows a more loaded reactor and the methane production is slightly higher. [ABSTRACT FROM AUTHOR]

This paper presents the study of the variations of the process parameters during transient conditions when substrates with different biodegradability were fed to a pilot-scale anaerobic digester. The two substrates used in the experiments were the mechanically selected organic fraction of municipal solid wastes and a blend of mechanically selected and separately collected organic fraction of municipal solid wastes. Two transient conditions were studied: the mesophilic--thermophilic passage and the increase of the organic loading rate in thermophilic conditions. It was shown that enhanced fractions of biodegradable substrates determined a greater perturbation of the system and needed longer time to reach a new stable condition. The volatile fatty acids concentration and their indirect measure, alkalinity determined upon pH 4, was the more useful parameter to understand the process behaviour because of its sensitivity to the process variations. [ABSTRACT FROM AUTHOR]

The paper deals with the performances of the mesophilic anaerobic digestion treatment of sewage sludge from a full scale BNR process without primary settling (nominally 300,000 PE). A relation between the activated sludge observed yields, Yobs, and the anaerobic digester performance was preliminarily found: for values of Yobs of 0.25 kgVSS/kgCOD the anaerobic digester specific gas production showed the best performances (0.22 m³/kgVSfed). This has to be confirmed with wider future studies. It was also shown the level of sludge pre-thickening to be reached for the self-sustaining warming of the digester also in wintertime. According to the energetic balance and to a comparison with an aerobic stabilisation process, it was pointed out as when a co-generation unit for heat and energy production was introduced about 3.4 kWh/PE y of energy were produced in the anaerobic digestion process. On the other hand, 4.3 kWh/PE y were spent if an aerobic stabilisation process was applied. The economic assessment, carried out on the basis of the energy balances, showed that the anaerobic digestion is always economically advantageous if compared to aerobic stabilisation processes, also for small WWTPs. According to the energetic evaluations an environmental balance was assessed, in terms of CO2 emissions. The difference between anaerobic and aerobic processes was about 5.3 kgCO2/PE y in favour of anaerobic processes application. [ABSTRACT FROM AUTHOR]

The paper presents preliminary experimental results which aim to demonstrate the possibility of producing easily biodegradable carbon from a two phase anaerobic digestion process for a biological nutrient removal process. Crystallization of phosphorus into struvite from the liquor of the digested flow is the second goal of the process studied. With this integrated process approach it is possible to double the amount of readily biodegradable COD entering the wastewater works by sewers and to recover up to 70% of phosphorus from the digested flow as struvite without using little amount of chemicals. Struvite granules are useful for agricultural purposes. [ABSTRACT FROM AUTHOR]

The production of volatile fatty acids (VFA) by anaerobic fermentation of municipal solid wastes was studied at pilot-plant level. A plug-flow reactor (80 1 total volume) without solid or liquid recirculation was employed to digest a mixture of two types of organic fraction of the municipal solid waste (OFMSW): OFMSW mechanically selected and OFMSW coming from a market of fruit and vegetables. The acidogenic process was studied at different retention times (between 2 and 6 days) in the mesophilic (37° ± 2°C) range of temperature. The VFA concentration obtained in the first valve of the tubular reactor ranged from 9.1 to 13.4 g 1-1 and in the outlet sludge oscillated between 11.8 and 23.1 g 1-1, increasing when increasing retention time from 2 to 6 days. A mathematical model of a continuous steady state plug-flow reactor proposed in a previous paper was used to represent the VFA production obtained experimentally. The results obtained when fitting the mathematical model with the experimental results were not sufficiently good. Some modifications of the initial model were considered. The best fitting of the results was obtained when the inhibition effect of the fermentation product on microorganisms growth was taken into account. The errors of this fitting, calculated by least squares, can be considered optimal for retention times between 4 and 6 days, while at shorter retention times the inhibition effect of pH and product makes the volatile acid production lower than the values obtained experimentally. [ABSTRACT FROM AUTHOR]

The study concerns the application of the semi-dry single phase thermophilic anaerobic digestion process to the organic fraction of municipal solid waste. The experiments were carried out using 3 m3 and 1 m3 CSTR pilot scale reactors. The process performance in terms of biogas yields, digester stability and kinetic aspects was studied, considering a progressive increase in the feed biodegradability, in order to evaluate the process behaviour changing from an undifferentiated collection of waste to a separate collection. This was carried out using blends of two different kinds of substrates: mechanically sorted organic fraction of municipal solid wastes (MS-OFMSW) and source sorted organic fraction of municipal solid wastes (SS-OFMSW). The study shows that OLR up to 6 kgTVS/m3d can beapplicable for the medium selected fraction (TVS/TS=0.7), while for the MS-OFMSW alone this limit can be doubled. The results obtained with SS-OFMSW alone suggest the use of the double phase process to givemore stable conditions. [ABSTRACT FROM AUTHOR]

Abstract: Anaerobic digestion model no. 1 model of international water association was applied to a full scale anaerobic co-digestion process for the treatment of the organic fraction of municipal solid wastes along with activated sludge wastes originating from a municipal wastewater treatment plant. This operation was carried out in a digester of 2000m3 in volume. It is operates at an average hydraulic retention time of 26.9 days with an average organic loading rate of 1.01kg TVS/m3 day, at a temperature of 37°C with an average gas production rate of 0.296m3/m3 day. The aim of the present study is to compare the results obtained from the simulation with the experimental values. The simulated results showed a good fit for pH, methane and carbon dioxide percentages, biogas volume, chemical oxygen demand, total volatile fatty acids, inorganic nitrogen and inorganic carbon. [Copyright &y& Elsevier]

Abstract: In this study the optimization of the biogas yield from anaerobic co-digestion of manures and energy crops was carried out using four pilot scale CSTRs under different operating conditions. The effect on biogas yield of the partial substitution of energy crops with agro-waste was also investigated. For each substrate used during the continuous trials, BMP batch assays were also carried out to verify the maximum methane yield theoretically obtainable. Continuous operation results indicated that the co-digestion of manures, energy crops and agro-waste was viable at all operating conditions tested, with the greatest specific gas production of 0.54m3/kgVSfed at an organic load rate of 2kgTVS/m3 rd consisting of 50% manure, 25% energy crops and 25% agro-waste on VS basis. No significant differences were observed between high and low loaded reactors suggesting the possibility of either improving the OLR in existing anaerobic reactors or reducing the design volumes of new reactors. [Copyright &y& Elsevier]

Abstract: The optimization of a two-phase thermophilic anaerobic process treating biowaste for hydrogen and methane production was carried out at pilot scale using two stirred reactors (CSTRs) and without any physical/chemical pre-treatment of inoculum. During the experiment the hydrogen production at low hydraulic retention time (3d) was tested, both with and without reject water recirculation and at two organic loading rate (16 and 21kgTVS/m3d). The better yields were obtained with recirculation where the pH reached an optimal value (5.5) thanks to the buffering capacity of the recycle stream. The specific gas production of the first reactor was 51l/kgVSfed and H2 content in biogas 37%. The mixture of gas obtained from the two reactors met the standards for the biohythane mix only when lower loading rate were applied to the first reactor, with a composition of 6.7% H2, 40.1% CO2 and 52.3% CH4 the overall SGP being 0.78m3/kgVSfed. [Copyright &y& Elsevier]