Recovery of Bio-energy and Resources from Organic Waste Streams in Established Trade Waste Customers

Industrial wastewater, also known as trade waste, can vary widely in its contents, properties, and volumes. Depending on the industry sector and internal processes of a business, each trade waste stream is unique. Organic compounds, acids, bases, metals, sulphur substances, herbicides, hydrocarbons, biologically active agents, and pathogens are some components that may be present in the liquid waste produced. These aspects mean that the uncontrolled release of trade waste can result in human health effects and environmental damage. To limit this damage, wastewater in metropolitan catchments is treated both on- site and in centralised treatment plants. In operating treatment plants, municipal water service providers monitor and collect data on the effluent as it leaves a business and set restrictions on discharge quality. This data is used as the basis of service fees, and fines can be levied for breaches, so on-site treatment before the point of discharge is incentivised. Unfortunately, this is seen primarily as a cost and a burden, with common treatment methods including neutralisation, destruction, and separation of restricted substances for disposal in landfill. This approach can be costly and a drain on resources, as well as only postponing or relocating environmental impacts. An alternative perspective on treatment sees trade waste as an opportunity, and the otherwise unwanted contents as valuable resources to reclaim through various specialised techniques, closing the loop of a circular economy and minimising greenhouse gas emissions. In particular, anaerobic digestion can convert organic compounds to biogas: a mixture of methane and other gases than can be used directly as a fuel or processed and sold. Resource recovery, despite the potential benefits, is less common at individual trade waste sites than traditional treatment techniques. To achieve sustainability goals and better water quality outcomes, it is in the interest of water service providers to assist trade waste customers in exploring the possibilities of resource recovery. The historical discharge data held by water corporations is seen as a possible key to identifying businesses who can shift to using anaerobic digestion to treat effluent. However, it is currently unknown how well this data translates to suitability for anaerobic digestion. Considering this unrealised potential, the broad objective of this work is to examine the plausibility of identifying targets for anaerobic digestion at established trade waste customers by understanding a service provider’s catchment. In the literature review, a holistic overview of resource recovery from industrial wastewater is presented. Generalised characteristics of effluent from different industry sectors are summarised, as well as example limits for certain factors. This literature review highlights various technologies and processes in the resource recovery space and evaluates their advantages and disadvantages. Proposed methods of classifying trade waste are also described. A research gap surrounding the efficacy of these schemes in identifying resource recovery prospects is identified, as they mostly remain theoretical, with actual implementation rare. Literature that focuses on the interactions between water service providers and their industrial customers regarding resource recovery wastewater treatment decisions is minimal and suggests hidden areas of1 opportunity. Although certain water chemistry parameters are readily measurable and identified by industry, the microbiome of trade waste, and the effects caused by non-biological treatments, is less clear. To answer whether data held by water utilities can be used to identify prospects for anaerobic digestion, a broad initial screening study was completed in Chapter 2. Several sites were selected, and pre-treated wastewater samples, corresponding to the historical data held by water service providers, were taken. Where possible, untreated samples were also obtained from the same sites, to give comparisons. Biomethane potential assays were carried out using these samples to find the quantity of biogas that may be possible to produce from the wastewater at each site. Methane yields varied between sites, reaching up to 357 mL/g VS for untreated waste, and 287 mL/g VS for treated waste. In general, at each site, raw wastewater that had not undergone any treatment yielded more methane than the discharge-ready treated trade waste. The historical data did not always reflect the actual composition of the treated effluent and had minimal and inconsistent correlation with the properties of untreated samples, a result of different trade waste customers utilising different treatments to different extents. Therefore, historical data was not a reliable indicator of anaerobic digestion potential, although a history of high sodium levels is likely to preclude suitability. Another aspect of the differences between samples across sites and treatment levels is the microbes found within them. With the aim of taking an inventory of microbes in trade wastes, and uncovering any links it has with treatment, industry, and suitability as anaerobic digestion substrate, Chapter 3 provides a snapshot of the microbial communities of the wastewaters studied previously. Using amplicon-based sequencing, unique taxonomy was found across the catchment. Treatment levels, industry, and gas yields were not found to be directly related to the original microbes present in a sample. Diversity varied between samples, with a slightly higher number of taxa found in treated trade waste when compared to the untreated wastewater at most sites studied. Although ultimately not directly applicable in a resource recovery context, this section provides a valuable record of microbes present in industrial wastewater. In finding that untreated wastewater can produce more biomethane, a new question is raised: are there internal waste streams that biological treatment can be concentrated on with positive effects? Chapter 4 consequently progresses to semi-continuous digestion of an internal waste-stream from a site identified as potentially suitable for anaerobic digestion, and its co-digestion with another high-yielding sample in the presence of a biosolids biochar additive. In this work, the suitability of source-separated strong waste streams was investigated, as well as considerations around interpreting BMP results in the context of practical applications, and microbiological and chemical changes as a digestion proceeds, particularly biogas production and volatile fatty acid accumulation. Process stability was found to be possible, with lower VFA accumulation and higher yields (0.43 L day-1 L-1 and 0.24 L day-1 L-1 of methane at 45 and 30 d HRT respectively) at a longer retention time. The microbial community shifted in similar ways between retention time treatments, and diversity of the microbes found in the reactors decreased over the course of the study.2 Methanosarcina was identified as the methanogenic genus most closely linked to overall methane production within this setup. The negative effect on process stability caused by changes in feed was also shown, emphasising the importance of process control. This work shows that historical data held by water service providers are only a small part of identifying suitable prospects for resource recovery processes. As this information is collected after the on-site treatment that works in competition to anaerobic digestion, and even then, does not capture all aspects of the water chemistry, it does not perfectly identify the potential for biogas production, an assumption unconfirmed until this work. In addition to this, certain source separated streams within a business may be even more suitable for digestion than the total combined effluent, shown in the later section of this thesis. As the presence of these fractions is masked by aggregation and on-site treatment, a more individualised approach is required, with examining high-level data only a first step. Biomethane potential tests oversimplify process design and control issues, so individual studies must also progress from batch tests to testing that more closely matches the final plant configuration through scale-up. Through closer examination of trade wastes, this work additionally provides a novel identification of the microbial ecology of various treated and untreated industrial wastewaters, and the dynamics of digesters using these substrates. Existing trade waste customers have been shown by this work to be a valid target when exploring anaerobic digestion as a resource recovery-focused wastewater treatment process, with potential benefits to circular economy and greenhouse gas emissions.