Anaerobic digestion (AD) technology can be employed for treating sewage sludge, livestock waste or food waste. Generally, the hydrolysis stage is the rate-limiting step of the AD processes for solid waste degradation. Therefore, physical, chemical and biological pre-treatment methods or their combination are required, in order to reduce the rate of such a limiting step. In this study, four methods (mechanical shredding, acid hydrolysis, alkaline hydrolysis and sonication) were tested to improve methane production and anaerobic biodegradability of different agro-food wastes and their mixtures. The kinetics of anaerobic degradation and methane production ofpre-treated individual wastes and selected mixtures were investigated with batch tests. Sonication at lower frequencies (37 kHz) proved to give the best results with methane productivity enhancements of over 100% in the case of pig manure and in the range of 10-47% for the other wastes assayed. Furthermore, the ultimate methane production was proportional, in all the cases, to the specific energy input applied (Es). Sonication can, thus, enhance waste digestion and the rate and quantity of biogas generated. The behaviour of the other pre-treatments under the conditions assayed is not significant. Only a slight enhancement of biogas production (around 10%) was detected for whey and waste activated sludge (WAS) after mechanical shredding. The lack of effectiveness of chemical pre-treatments (acid and alkaline hydrolysis) can be justified by the inhibition of the methanogenic process due to the presence of high concentrations of sodium (up to 8 g l(-1) in some tests). Only in the case of WAS did the acid hydrolysis considerably increase the biodegradability of the sample (79%), because in this case no inhibition by sodium took place. Some hints of a synergistic effect have been observed when co-digestion of the mixtures was performed.
The purpose of this work was to study the anaerobic co-digestion of all the by-products generated during the processing of used vegetable oils (UVO) with the objective of proposing a solution for the treatment of these wastes. More specifically, this study investigates the advantages of two-phase anaerobic digestion for treating a mixture (1/5 v/v) of UVO processing wastes (OW) and pig manure (PM) using two semi-continuous digesters operated at mesophilic temperature (37 ± 1°C). The experiments were conducted at hydraulic retention time (HRT) of 0.5, 1, 2 and 4 days in the first stage (acidifier) and at HRT of 11.5, 15, 18 and 20 days in the second stage (methaniser). The results revealed that the HRT had a high influence on the soluble chemical oxygen demand (sCOD) and total dissolved solids (TDS) removal. The maximum total sCOD removal efficiency of 86.4 % and TDS removal efficiency of 81.9 % was achieved at 20 days of global HRT. The maximum biogas production of 0.65 m 3 per kilogramme of volatile dissolved solids (VDS) was removed, corresponding to a methane production of 0.42 m 3 CH 4 kg -1 . VDS removed (65 % CH 4 ) was also achieved at 20 days of HRT. The two-phase digestion system showed good stability, which was mainly attributed to the strong buffering capacity with the two-phase system and the high alkalinity from PM when co-digested with OW. The results obtained from this study provide fundamental information for scaling up a high-performance two-phase anaerobic system in the future.
Rice straw (RS) is one of the most abundant wastes generated in Valencia (Spain). Traditional waste disposal methods are harmful to the environment. The straw burning emits large amounts of toxic air pollutants and the straw burying produces uncontrolled anaerobic fermentation in the soil. The aim of this research was to evaluate the feasibility of cow manure anaerobic co-digestion with RS in a semi-continuous plug flow pilot-scale reactor. Three different periods of co-digestion were carried out as the RS dose was increased. When the addition of RS was 1, 2 and 5% (on weight basis) the biogas productivity increased by 4, 28 and 54% respectively. Furthermore, economic and energy feasibility were analysed considering the logistics cost of the RS (baling, collection, crushing and transportation). Two different scenarios were analysed. In scenario 1, the anaerobic co-digestion process was considered that take place in a new biogas installation, and in scenario 2 the process was considered that take place in a biogas plant already in operation. In scenario 1, the cow manure co-digestion with 2% of RS in a biogas installation of 500 kW showed the best economic analysis (net present value of 13.23%). In scenario 2, the results showed that the maximum distance between the rice field and the biogas plant that produces a positive economic balance was less than 95 km (2% RS) and 74 km (5% RS). In the case of the addition of 1% RS the economic balance is negative. Energy balance is positive in the three mixtures analysed.
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