The combination of rising energy consumption in the U.S. and sustained growth of developing countries has made clear the importance of developing an energy source that is renewable and minimizes greenhouse gas emissions. The use of algae as an energy source can satisfy both of these criteria, but the current focus on developing it as a biofuel requires a significant amount of energy input, making it not yet economically feasible. This research combines a promising energy source with a decades-old wastewater treatment technology to generate biogas by combining the anaerobic digestion of algae and wastewater sludge. Bench-scale anaerobic digesters were setup with various proportions of the microalgae Scenedesmus quadricuada and thickened waste activated sludge (TWAS) and their biogas production was evaluated. In addition, the effects of operational parameters, such as temperature and alkalinity, on biogas production and residual characteristics were investigated. Biogas production for the various algae and TWAS combinations ranged from 0.46 to 0.72 mL per mg of volatile solids (VS) digested, while VS and chemical oxygen demand (COD) were reduced on average, 47 and 50%, respectively, at 35°C. Total coliform (TC) and fecal coliform (FC) concentrations saw at least a one log reduction after digestion, allowing the digestant to meet the USEPA requirements for classification as a Class B biosolid and its use in certain land applications. The digestant had nitrogen and phosphorous levels in the range of 5 to 19% as N and 5 to 15% as P, respectively, putting it in the range of commercial fertilizer levels. It was also determined that decreasing digestion temperatures from 35°C produced significantly less biogas, while adjusting the amount of initial alkalinity in digesters did not have a significant effect on biogas production. From these results, anaerobically digesting algae along with wastewater sludge can be utilized as a feasible method to harness the energy potential of algae. Although some of this potential remains locked up in the undigested portion, its synergy with wastewater treatment plants (WWTPs) cannot be overstated. Growing algae using existing waste streams at WWTPs such as CO2 and effluent wastewater highlights this technology's ability to transform waste into a valuable commodity without enormous new infrastructure investment..

Increasing demand for energy coupled with concerns over limited fossil fuel reserves and apprehensions over their contributions to greenhouse gas emissions have made the search for low carbon energy sources a high priority. Algal biomass could serve as an alternative source of renewable biofuels. Research efforts to date have primarily focused on the production of algal biofuels through lipid extraction, which involves high temperature and high pressure, resulting in an energy intensive process. In this research, the use of algal biomass as a supplementary feedstock to anaerobic digesters for the production of methane gas is evaluated. To test the potential of algal biomass as a supplementary feedstock, labscale anaerobic digesters are set-up. The methane gas production of various combinations of thickened waste activated sludge (TWAS) and algal biomass is investigated. Chlorella vulgaris (C. vulgaris) is used as representative microalgae. In addition, the effects of operational parameters, such as biomass loading, temperature and alkalinity, on biogas production are investigated. The results show that the biogas production for all biomass loading combinations of C. vulgaris and TWAS ranged from 0.47-0.57 mL per mg volatile solids (VS) digested. On average, VS and chemical oxygen demand (COD) were reduced 48 and 38%, respectively, at 35°C. Average total coliform (TC) and fecal coliform (FC) concentrations of 6.3x104 and 1.0x104 CFU per gram of total solids (TS), respectively, were measured in the digested waste at 35°C. Thus, the residual meets the USEPA requirements for pathogen reduction (FC 2x106 CFU per g TS) and vector attraction reduction ( 38% reduction in VS) for land application. The total nitrogen and phosphorus content of the residual was determined to be in the range of 9-17% as N and 3-7% as P (7-16% as P2O5), respectively, revealing its potential value as a fertilizer. It was also observed that decreased digestion temperatures resulted in lower biogas yields, while initial alkalinity in digesters did not appear to affect biogas production. From the results of the research, it can be inferred that algae can be co-digested with wastewater sludge, or by itself, to produce methane gas at wastewater treatment plants (WWTPs). This suggests that algae can be utilized as an energy source through anaerobic co-digestion with wastewater sludge. This is significant because algae can be grown with the nutrient and CO2 contained in waste streams at WWTPS, thereby minimizing the release of nutrients and effluent water to the environment. This reduced nutrient load results in treatment cost savings, while the reduction in effluent discharge decreases environmental pollution.

Algae that are grown in wastewater treatment lagoons could be an important substrate for biofuel production; however, the low C/N ratio of algae is not conducive to anaerobic digestion of algae with economically attractive methane production rates. Increasing the C/N ratio in anaerobic, laboratory scale, batch reactors by blending algal biomass with sodium acetate resulted i increased methane production rates as the C/N ratio increased. The highest amount of methane was produced when the C/N was 21/1. When the C/N was 24/1, the biogas production rate decreased. Batch experiments were done to evaluate the effect of optimizing the C/N ratio on methane production from algae and to identify the most essential information needed to conduct research on co-digestion of algal biomass using the continuous, high-rate, up-flow anaerobic sludge blanket (UASB) reactor system. Based on the results obtained from batch reactor experiments, anaerobic co-digestion of algal biomass, obtained by continuous centrifugation from the Logan City, Utah, 5th stage wastewater treatment lagoon, and sodium acetate was conducted using laboratory scale UASB reactors with the C/N ratio in the feedstock adjusted to 21/1. Duplicate, 34 L UASB reactor systems were built of poly(methyl methacrylate). Both reactors were seeded with 11 L of anaerobic sediment from the 3rd stage lagoon. The pH of the feedstock was adjusted to the neutral range. The feedstock was initially introduced at a low organic loading rate of 0.9 g/L.d with a hydraulic retention time (HRT) of 7.2 days and then increased up to 5.4 g/L.d and a HRT of 5.5 days. These organic loading rates corresponded to an initial influent chemical oxygen demand (COD) of 6.25 g/L and increased to 27.2 g/L. Methane production increased from 270 mL/g to 349 mL/g COD biodegraded. COD removal efficiency was 80% and biogas methane composition was 90% at steady state. Algal biomass contributed 33-50% of the COD in the feed stock depending on the COD of the algae paste from centrifugation. The shortest HRT at which steady state was not affected was 5.5 days. At lower HRT all monitored parameters showed a slight decrease after the 75th day of operation.

This volume in the Energy from Wastes Series covers the area of methane production from agricultural and domestic wastes. Principally this involves the conversion of excreta and other organic effluents to a valuable gaseous fuel plus, in many cases, a useful sludge for fertiliser or feedstuffs. Dr Hobson and his colleagues have written a comprehensive text on the principles of microbiological processes and the biochemistry of anaerobic digestion, embracing the design of digesters with examples of current working installations. The potential for anaerobic digestion of wastes as diverse as sewage to fruit processing effluents is also reviewed. This work should be of interest to all who have to manage organic waste treatment and disposal, as well as to a wider readership who wish to know more about methane production by anaerobic digestion. ANDREW PORTEOUS v Preface The production of methane, or more exactly, a flammable 'biogas' containing methane and carbon dioxide, by microbiological methods ('anaerobic digestion') is not new. The reactions have been in industrial use for over a hundred years, but only in sewage purification processes. In some times of national stress, such as war-time, the microbiological production of gas purely for fuel has been investigated, but with the resumption of plentiful su pplies of fossil fuels the investigations have faded awa y.

"Full scale anaerobic co-digestion of fruit and vegetable waste (FVW) and municipal wastewater primary sludge significantly increased biogas production. Digester operation remained stable. Undigested FVW was visible in dewatered sludge suggesting that FVW should be added to the first stage digester to prevent short-circuiting and increase the hydraulic retention time (HRT) of the FVW. Batch lab results confirmed that co-digestate addition to first stage sludge (FSS) is preferred to second stage sludge (SSS). FSS produced significantly more methane (514 ± 57 L CH4 kgVS−1 added) than SSS (392 ± 16 L CH4 kgVS−1 added). In a related study, combined alkaline and ultrasonic pre-treatment of thermomechanical pulp mill sludge (PMS) significantly increased the soluble TS, VS and COD of the PMS over non-treated sludge. Pre-treatment did not significantly improve biogas production over 28 d, but did increase VS reduction, and the initial rate of methane production. Overall, biogas production from PMS was inconsistent."--P. ii.

Fat, oil, and grease residues, food particles, solids and some kitchen wastewaters are collected in grease traps which are separate from the municipal wastewater stream. Grease traps are emptied periodically and grease trap waste (GTW) is hauled for treatment. This dissertation focuses on anaerobic co-digestion of un-dewatered (raw) GTW with municipal wastewater treatment sludge (MWS) at wastewater treatment plants. In particular, this research focuses on the biochemical methane potential of un-dewatered GTW as well as the stability and performance of anaerobic co-digestion of MWS and un-dewatered GTW. A set of modified biochemical methane potential tests was performed to determine the methane potential of un-dewatered GTW under mesophilic conditions (35 °C). Methane potential of un-dewatered GTW in this study was 606 mL CH4/g VS [subscript added] which is less than previously reported methane potentials of 845 - 1050 mL CH4/g VS [subscript added] for concentrated/dewatered GTW. However, the methane potential of un-dewatered GTW (606 mL CH4/g VS [subscript added]) was more than two times greater than the 223 mL CH4/g VS [subscript added] reported for MWS digestion alone. A comprehensive study was performed to determine the stability and performance of anaerobic co-digestion of MWS with un-dewatered GTW as a function of increasing GTW feed ratios. The performance of two semi-continuously fed anaerobic digesters at 35 °C was evaluated as a function of increasing GTW feed ratios. Anaerobic co-digestion of MWS with un-dewatered GTW at a 46% GTW feed ratio (on a volatile solids basis) resulted in a 67% increase in methane production and a 26% increase in volatile solids reduction compared to anaerobic digestion of MWS alone. On the other hand, anaerobic co-digestion of un-dewatered GTW resulted in a higher inhibition threshold (46% on VS basis) than that of dewatered GTW. These results indicate that using un-dewatered GTW instead of dewatered GTW can reduce the inhibition risk of anaerobic co-digestion of MWS and GTW. Recovery of the anaerobic digesters following upset conditions was also evaluated and semi-continuous feed of digester effluent into upset digesters yielded of the biogas production level of the undisrupted digestion. Finally, a mathematical model was used to describe the relationship between methane potential and GTW feed ratio on a VS basis. The results of this research can be used to predict methane production and identify suitable GTW feeding ratios for successful co-digestion of un-dewatered GTW and MWS.