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Sewage sludge is widely used as an important source for biogas production through digestion. Developing the high performance processes has a significant goal in order to promote energy efficiency and reduce the cost sewage sludge treatment. The problem of sewage sludge disposal is becoming top one which almost cost 50 % of running fee for a municipal wastewater treatment plant. This paper basically introduces three methods to improve the conventional digestion. However, they enhance the conventional digestion from different aspects. For examples, Two-phase anaerobic digestion enables to exhibit the merit of thermophilic anaerobic digestion and avoid the weak points of conventional digestion regarding odor problem. In two-phase anaerobic digestion, the acid and methane producing stages are separated. Extended solids retention time is an approach to separate the hydraulic retention time and solids retention time in an anaerobic digester by using recycle thickening. This method could benefit further de-composing the organics and increase methane formation. Dewaterability is the final step of anaerobic digestion process. Enhancing this part of process is an efficient way to increase the solid content of sludge that would reduce the transportation costs. In a nutshell, no matter on saving cost or energy perspectives, these three methods all promote biogas production efficiency up to a better performance, but various requirement of energy and cost are demanded. The paper displays and compares the advantages and disadvantages among three methods. There is no certain answer to which method is the best one; however, they can be chose to enhance digestion in different condition.

Sewage sludge is treated with the biological process of anaerobic digestion in which organic material of a substrate is degraded by microorganisms in the absence of oxygen. The result of this degradation is biogas, a mixture mainly of methane and carbon dioxide. Biochemical Methane Potential tests are used to provide a measure of the anaerobic degradability of a given substrate. This study aims to determine the methane potential in Sjöstadsverket’s sludge this will moreover determine the viability of recycling the digested sludge back into the anaerobic system for further digestion. Batch digestion tests were performed in both Sjöstadsverket’s (S1) and Henriksdal’s (H2) sludge, for a reliable comparison. An inoculum to substrate ratio of 2:1 based on VS content was used and BMP tests presented results that S1 and H2 in the 20 days of incubation produced 0.29 NLCH4/gVS and 0.33 NLCH4/gVS respectively. A second experiment considering the same amount of substrate (200ml) and inoculum (200ml) for each sample, showed that Control S1 had a higher methane potential than Control H2, 0.31 NL/gVS and 0.29 NL/gVS respectively. All the samples containing Sjöstadsverket’s inoculum presented a higher volume of total accumulated gas (measured in Normal Liters), however methane potentials are low. Results demonstrated that methane production in samples S1 and Control S1 was originating from the grams of VS in the inoculum itself after depletion of all the soluble organic material in the substrate. This suggested that Sjöstadsverket’s sludge can endure a higher organic load rate and that the digested sludge still has potential to produce biogas, hence the recycling of this can enhance the biogas production in the digestion system.

Anaerobic digestion is a complex process, dependent on multiple factors, which can be made more stable and efficient with the help of process models. A more stable process could reduce biogas torching, which increases the amount of renewable energy available for heat- and electricity production and vehicle fuel.  Fluctuations in the produced biogas at Uddebo WWTP in Luleå, Sweden, has been observed, where imported industrial waste was suspected to be the cause. The purpose of this master thesis was to increase the predictability of the biogas production at Uddebo WWTP by creating a process model based on historical data. The historical data consisted of quantitative data of the substrates loaded into the anaerobic digestor and biogas collected from the digestor.  The results showed that the calculated error between modelled and measured biogas production was <20%, which was considered good. However, due to the demarcation of this report, many variables were ignored in the creation of the model. Further research is therefore a requirement for the model to function properly:  The error should be reduced to <10%, This could be done by increasing the period and resolution toinclude and statistically secure fast and seasonal variations.  make qualitative modelling easier, i.e., prediction of the biomethane amount in the biogas.  The industrial liquid waste storage should be included in the calculation.   A more detailed analysis of all substrates is needed to make the comparison between the literature based and measured production more realistic.  Expanding the demarcation of the model to also include the environment in the digestor. This could also increase the total amount of biogas produced.

Digestate produced during anaerobic digestion of food waste is recognised as a good alternative to mineral fertilizer which could also be used to amend soil properties. This has conventionally been applied directly and unprocessed to nearby farms or processed and transported elsewhere. The latter option has gained recognition due to environmental restrictions coupled with soil nutrient management objectives but is an expensive venture. With increasing biogas production and AD plants across Europe, production of digestate has however exceeded its demand. Improving the dewaterability of the digestate has the benefit of reducing the cost and time of processing and handling. The principal aim of this experiment was to enhance the dewaterability of food waste digestate by the addition of pulp fibre to the AD process. In doing so, the study also investigated the effect of co-digestion of food waste and pulp fibre on the performance and stability of the digestion. Source separated food waste was digested at OLR of 3.5 ±0.1 g VS/L*d-1 for 163 days in 3 CSTRs with a working volume of 6L at HRT OF 23-26 days. Soft- and hardwood pulp fibres were added to 2 designated digesters for 104 days and increased stepwise at OLR of 0.5 ±0.1 g VS/L*d-1 PF until 1.5 ±0.1 g VS/L*d-1 PF with the 3rd digester serving as a control. 3 other post-digesters, each with a working volume of 1.41L were operated for 104 days with sludge from the 3 main digesters serving as inoculum and substrate. This was run at HRT of 7 days.  Pulp fibre addition of 1.5 ±0.1 g VS/L*d-1 OLR to 3.5 ±0.1 g VS/L*d-1 food waste increased the total biogas and methane production to 35-40% and 21-32% respectively. Though recording a higher biogas production, the corresponding specific methane production from the fibre addition was 12-8% lower than food waste digestion. Analysis of the digestate from post digestion showed that CST increased linearly from 595 ±13s for food waste digestate to 962 ±19s for pulp fibre addition. The experiment established a positive correlation between CST and organic matter content. Suspended solids increased from 128 ±10 mg/l for FW digestate to 177 ±12 – 237 ±10 mg/l for fibre addition. Addition of kraft pulp fibre types did not enhance the dewaterability of the digestate. However, the total methane production was enhanced by the addition of pulp fibre. / <p>Parts of the thesis is scheduled to be published in a scientific journal</p>

Sole dependence on fossil fuel and the concomitant environmental concerns could be minimized through the optimization of green energy generation from the growing volume of onfarm organic wastes. In this mesophilic study, green energy, mainly methane, was optimized through the solid-state anerobic co-digestion (SSAD) of two on-farm organic wastes (dairy manure with corn stover). Factors considered to achieve the improved methane yield under a total solids of 16% were particle size of corn stover (0.18 – 0.42 and 0.42 – 0.84 mm), alkaline pretreatment type (thermo-chemical and wet state), alkaline-pretreatment reagent (NaOH, NH4OH, and Ca(OH)2) used for the corn stover, and the magnetite nanoparticles(20, 50, and 75 mg/L) thereafter added to the treatment with highest methane yield. Kinetic models were used to describe some of the high methane yield as well as the environmental impact investigated with life cycle assessment. Results indicated that corn stover with particle size 0.42 - 0.84 mm blended with dairy manure under a C/N of 24 had the highest methane yield (106 L/ kgVS) under 60 days retention time. After pretreatment of the 0.42 - 0.84 mm corn stover with the three different alkaline reagents, methane yield improved under this wet state pretreatment relative to thermochemical. For instance, calcium pretreated corn stover blended with dairy manure (CaW) had the highest methane yield (176 L / kgVS) under a reduced retention time (79 days), overcame potential volatile fatty acids accumulation and digester upset relative to other pretreated treatments. Furthermore, addition of 20 mg of the nanoparticles to the CaW treatment further enhanced methane yield (191 L / kg VS), minimized digester upset, and reduced retention time to 52 days. Suitable process parameters for methanogenic activities were 0.1 - 0.5 for VFA/Ammonia and VFA/Alkalinity ratios. Free ammonia concentration between 258 – 347 mg/L does not affect methanogenic activities. Environmnetal impact aseessment indicated that pretreatment negatively influenced human health factors and eutrophication potentials though reduced ozone depletion, global warming potential, and smog potentials. The solid-state of dairy manure co-digested with corn stover has the potential to improve green energy generation that could complement fossil fuel and address waste management challenges.

Fats, oil and grease (FOG) are generated in large amounts by cooking and food processing. Anaerobic co-digestion with municipal sewage sludge has proven to be one of best alternatives for FOG disposal due to its high potential for biogas production. However, excessive addition of long chain fatty acid, the major content of FOG, has been reported to have inhibitory effects on the anaerobic digestion process and to cause operational challenges. In this study, high purity long chain fatty acids (LCFAs) including linoleic acid, oleic acid, and a mixture of oleic acid and stearic acid were added to laboratory completed mixed anaerobic digesters. The performance of the digesters in terms of solids destruction, COD degradation, LCFAs accumulation and gas production was investigated. After reaching steady state, a large amount of palmitic acid was found in the reactors with oleic acid addition and mixture of stearic and oleic acid addition. In the meantime, no palmitic acid increase was observed in reactors where linoleic acid was added. A better solids and COD reduction and a higher biogas production were observed in reactors with higher LCFAs addition. For reactors with the same dosage of LCFAs addition, linoleic acid addition resulted in the greatest improvement in digester performance; the mixture of stearic acid and oleic acid achieved the least increase in biogas production and solids and COD reduction. A high concentration of both palmitic and stearic acid in the reactors with oleic acid addition and with 20% mixed acid addition was observed. In contrast, linoleic acid and 30% mixed acid addition did not lead to a greater palmitic or stearic acid concentrations. Up to 30% of pure linoleic acid, oleic acid and mixed acid addition are able to enhance the performance of anaerobic digesters. It is recommended that the dosage of oleic acid be below 30% to avoid LCFAs accumulation and to increase reactor stability. / Master of Science

Landfill leachates and thermal hydrolysis pretreated anaerobic digestion centrate can quench UV light at publicly owned treatment works (POTWs). Increased eutrophication, has led to tightening of nutrient discharge limits in some regions of the country. Biologically recalcitrant organic nitrogen, adds to effluent nitrogen making it difficult to meet these requirements. The study aimed at characterizing landfill leachate and centrate fractions to develop an understanding that might help design on-site treatment methods. Leachates varying in on-site treatment and ages were fractionated on basis of hydrophobic nature. Humic substances were the major UV light quenching fractions. Majority of the humic substances were > 1 kDa molecular weight cut off (MWCO) indicating that membrane treatment might be effective for UV quenching substances removal. UV absorbing substances were found to be more bio-refractory than organic carbon. Significant decrease in humic substances with long term landfilling indicated that age was important in determining the potential for leachates to impact the UV disinfection. Organic nitrogen was observed to be hydrophilic in nature (mostly < 1 kDa). Proteins which are easily biodegradable contributed around one-third of the organic nitrogen. For thermal hydrolysis centrate, the optimum treatment depended on particle size and hydrophobic nature. Biological treatment was observed to be more effective for the removal of organic matter and UV254 quenching substances for fractions < 300 kDa. Biological treatment had little impact on organic nitrogen. Coagulation-flocculation is an effective treatment for higher molecular weight (MW) fractions whereas a membrane bioreactor would be more suitable for smaller MW fractions. / Master of Science

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Volatile Fatty Acids (VFAs) are a by-product when producing methane through anaerobic digestion (AD). Due to their many uses as building block chemicals, it is of interest to look into ways to optimize anaerobic digestion toward VFA production instead of methane generation. This report will focus on different parameters to produce VFA from food waste (FW), primary sludge (PS), and digested sludge (DS) in different ratios. In this project, three different experimental sets were run over a period of 25 days. The three different sets were 100 % FW, 100 % PS, and 50/50 % FW and PS mixture. Reactors were adjusted to an initial pH-value of 10 and then sparged with nitrogen to create an anaerobic environment. Measurements of the pH were done by sampling at each retention time. Gas chromatography (GC) was used at the end of the project to determine the concentrations of the VFA in the samples. The results showed that, with pH 10, a retention time of 15 days and using only FW, was optimal for VFA production as it gave the highest total concentration of 14.03 g VFA/L. Acetic acid was found in the highest concentration in all ratios. A mixture of FW and PS had an optimal retention time of 12 days, but did not generate as high concentrations of VFAs as only using FW, with a maximum concentration of 9.34 g VFA/L. Using only PS generated even lower concentrations, with a maximum of 5.33 g VFA/L, but did not start decreasing during the experimental run, and no clear conclusion can, therefore, be drawn.