The objective of the study was to identify the impact of co-digesting clarifier skimmings on the overall methane generation from the treatment plant and additional energy value of the increased methane production. Biogas production from co-digesting clarifier skimmings and sewage sludge in pilot-scale fed-batch mesophilic anaerobic digesters has been evaluated. The digester was fed with increasing quantities of clarifier skimmings loads: 1.5, 2.6, 3.5 and 7.0 g COD equivalent/(L·d) (COD: chemical oxygen demand). Average volatile solids reduction of 65% was achieved in the scum-fed digester, compared with 51% in the control digester. Average 69% COD removal was achieved at highest scum loading (7 g COD eq/(L·d)) with approximate methane yield of 250 L CH(4)/kg COD fed (4 ft(3)/lb COD fed). The results show that scum as co-substrate in anaerobic digestion systems improves biogas yields while a 29% increase in specific CH(4) yield could be achieved when scum load is 7 g COD eq/(L·d). Based on the pilot-scale study results and full-scale data from South East Water Pollution Control Plant and Northeast Water Pollution Control Plant the expected annual energy recovery would be approximately 1.7 billion BTUs or nearly 0.5 million kWh.
Recent research efforts demonstrated an increase in fecal coliform counts in anaerobically digested biosolids after dewatering. Variety of bacteria enters viable but nonculturable (VNC) state as a survival response when exposed to environmental stress. Increase in coliform concentration after digestion and dewatering processes have been attributed to cells going into a viable but non-culturable state implying that traditional coliform enumeration methods are not sufficient to determine number of viable cells. Therefore, this research has been undertaken to develop a method for rapid and accurate quantification of viable but non-culturable pathogens in biosolids via monitoring and quantifying stress-related genes in Salmonella sp. The proposed method has the potential to allow accurate detection of pathogens in biosolids even when the cells are non-culturable due to environmental stress. The research proposed identification of stress related genes in Salmonella when cells are exposed to heat for different durations by using available Salmonella microarrays. In the context of this study the identified stress genes can be quantified through reverse transcription, complementary DNA (cDNA) synthesis, and amplification of cDNA via quantitative reverse transcription polymerase chain reaction (qRT-PCR). Then quantity of mRNA can be correlated to cell viability and cells ability to grow, i.e., their culturability. Development of a novel approach to understand the pathogen behaviour in biosolids is key to ensure low public health risks from biosolids. Nevertheless, the initial results suggest that intact RNA isolation from biosolids is still challenging task.
This study focused on the evaluation of biosolids management systems (BMS) from a natural resource consumption point of view. Additionally, the environmental impact of the facilities was benchmarked using Life Cycle Assessment (LCA) to provide a comprehensive assessment. This is the first study to apply a Cumulative Exergy Extraction from the Natural Environment (CEENE) method for an in-depth resource use assessment of BMS where two full-scale BMS and seven system variations were analyzed. CEENE allows better system evaluation and understanding of how much benefit is achievable from the products generated by BMS, which have valorization potential. LCA results showed that environmental burden is mostly from the intense electricity consumption. The CEENE analysis further revealed that the environmental burden is due to the high consumption of fossil and nuclear-based natural resources. Using Cumulative Degree of Perfection, higher resource-use efficiency, 53%, was observed in the PTA-2 where alkaline stabilization rather than anaerobic digestion is employed. However, an anaerobic digestion process is favorable over alkaline stabilization, with 35% lower overall natural resource use. The most significant reduction of the resource footprint occurred when the output biogas was valorized in a combined heat and power system.
Despite stringent monitoring, an increasing number of studies have recently showed evidence of pathogen survival in stabilized biosolids. The magnitude of such pathogen reactivation exceeded the current regulatory limits and it seems that the phenomenon is not limited to a few underperforming facilities. The viable but non-culturable (VNC) state of pathogens has been proposed within the last few decades as a survival mechanism for certain prokaryotic microorganisms, including pathogens, when they encounter unfavorable environmental conditions (Koch, A, 1971;Oliver, J.D., 1993). Several of the recent studies attributed the regrowth of indicator organisms in biosolids to the possible VNC state that microorganisms assume under unfavorable conditions during stabilization or exposure to chemical disinfectants (Higgins et al., 2007;Iranpour and Cox, 2006;Jolis, 2006;Manios, et al., 2006;Qi et al., 2007). The objective of this research is to review the current literature on pathogen regrowth in biosolids and to investigate the relationship between the expression of stress-related genes in Salmonella and cell viability. The study is based on the fact that the quantity of messenger ribonucleic acid (mRNA) in living organisms is proportional to their metabolic activities; i.e., nutritional state, and therefore the mRNA is expected to decrease significantly when organisms are under stress. A list of shock-response and housekeeping genes for Salmonella has been identified by a literature review. Several different protocols and readily available kits for the extraction of mRNA from the thermally treated biosolids have been studied for the best extraction yield. The next step of the research is to expose biosolids samples to heat stress and monitor changes in mRNA concentrations as a function of time. Detection of mRNA will be accomplished through first reverse transcription of mRNA pieces into complementary DNA (cDNA) and amplification of cDNA through q-PCR. Finally, the quantity of genes expressed will be correlated to cell viability by determining the number of cells that maintained cell integrity and ability to grow. The preliminary results suggest that gene expression has the potential to provide accurate and quantitative estimates of viable cells in biosolids. However, extraction of clean and intact mRNA from a complex matrix such as biosolids remains a challenging step for routine use of this novel approach.
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