Water Journal : Water Journal April 2012
wastewater treatment technical features 140 APRIL 2012 water The digestion process begins with bacterial hydrolysis to break down organic polymers such as carbohydrates, fats, proteins, amino acids, fatty acids and sugars to make them available for other bacteria. The sugars, fatty acids and amino acids are eventually converted into methane and carbon dioxide (see Figure 1). The key thing to note from this diagram, and the anaerobic digestion process, is that it is the carbohydrates, fats, proteins, sugars, fatty acids and amino acid content which are the critical "food source" for methane production. Therefore, waste streams that contain these compounds in higher concentrations will result in higher biogas production. Available Anaerobic Digestion Technologies There are a number of technologies that can be considered for energy production through the generation of biogas in WWTPs. The processes available include various types of anaerobic digestion, either in large tanks (digesters) or via low-energy covered anaerobic lagoon systems. The key strengths and weaknesses of the available anaerobic digestion technologies are summarised in Table 1. Preferred Waste Sources To aid biogas generation at smaller WWTPs the addition of non-sewage waste streams can be beneficial. Alternatively, wastewater flows can be increased through sewer mining of additional sewage flows in the catchment (if they are available), or from trucked wastewater sludges. In many catchments additional wastewater is not available and, therefore, non-sewage waste streams should be considered for co-digestion to improve biogas production. These non- sewage wastes should be high-strength wastes (HSWs) and would be trucked into site in solid, semi-solid or liquid form. Most wastes currently being sent to landfill can be considered for co- digestion. However, some wastes are preferred over others for biogas production, typically dependent on their energy content and physical characteristics. Wastes that are typically available and have been considered for these types of facilities include: • Municipal and industrial process wastewater/sludge; • Food and drink manufacturer process waste (i.e. beverage, meat processing, dairy, brewery or winery); • Paper/pulp waste; • Greasy waste/fats, oils and greases (FOG) (i.e. grease trap pump-outs); • Residential food and green waste (via trucked collection); • Residential/commercial food waste (organics rubbish bins); • Food waste (from markets or supermarket chains). Greasy wastes, food waste and FOG are preferred over wastes such as lawn clippings or grape skins from winery waste as the components of these (skin/ stalks) can be difficult to digest. However, it should be noted that solid food waste can have considerably higher pre- treatment requirements when compared to other waste types and this should be considered when evaluating the cost effectiveness of its use in co-digestion. Many non-sewage wastes are seasonal in production (i.e. dairy processing and meat production such as lamb processing) and there are a number of drawbacks in using seasonal waste sources. One of the key concerns is the over-sizing of facilities. Anaerobic digestion requires continuous feed (quality and flow), therefore the required storage would need to be sufficiently large to store seasonal waste when it is generated, and then slowly feed the waste continuously into the digestion process over the low season. This increases the potential of the digestion process becoming less stable and the likelihood of foaming occurring as the loading rates are changed. The additional capital investment required for the larger storage and processing facilities is also significantly higher than any benefit realised from the addition of a seasonal waste stream. The preferred waste material for co-digestion will also depend on the availability of the waste at an economic haul distance and expediency (i.e. can it be held at the source until needed or must it be accepted at the whim of the generator), as well as the biogas yield potential of that waste stream. Implementation of a Biogas Production Facility There are a number of other considerations when investigating the feasibility of implementing a biogas production facility. Some of these include planning and regulatory approvals, legislation, disposal of the by-products (digestate and biosolids), odour control and, of course, the expected lifecycle financial benefits and investment payback period. An example of HSW unloading at an anaerobic digestion and biogas generation facility. Simultaneous unloading of HSW tanker trucks.
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