Large systems (>150,000 EP) can justify the large capital expenditure to implement advanced anaerobic digestion technologies, including thermal hydrolysis. This is environmentally very beneficial, in terms of greenhouse gas emissions, compared to the most common approach of aerobic digestion, and costs are of a similar order of magnitude. However, much of the costs previously directed at disposal, are now directed to capital depreciation, with the benefits of decreased operating cost, more predictable whole of life costs, and benefits to the environment. In any case, centralised high technology biosolids treatment systems are a real option for long-term beneficial use of biosolids.
The picture is bleaker for small and medium systems, and there are no options that are emissions negative, or neutral. To address this; (a) smaller reactors need to be built at lower prices; (b) smaller scale cogeneration options need to be explored; (c) degradability needs to be improved; and (d) a class A biosolid should be produced. We are doing research to address the last two points, using a new technology that treats the incoming activated sludge at 60+C for a nominal 2 days to enhance degradability and improve final biosolids quality.
Biosolids sources and production levels
Solids streams are produced throughout wastewater treatment plants, with the major streams originating from concentrated sewage solids, in primary clarifiers, and excess activated sludge, from the secondary clarifiers. Modern plants are producing far more activated sludge than primary sludge. In general, each treatment plant will produce approximately 50-65 grams dry solids per person, per day (5). This depends largely on biosolids handling methods and wastewater process type, rather than upstream consumer habits (in contrast with total sewer flows). Costs of treating, managing, and disposing this solids stream ranges from (in the US and Europe), 25%-50% of total wastewater treatment plant costs (6). Australian treatment plants produce even more biosolids, as local secondary (activated) sludges are known to have poor dewaterability with belt filter press cake solids of 12%-18%, instead 20%-30% in other developed nations (4). The reason for this has not been adequately explained, but could be related to a inert solids input, and/or higher bound water fraction (7). In table 1, some basic guidelines are given for the different sized plants. Costs especially are indicative only, and given localised environmental, social, and logistic constraints, can double or triple.