How to improve anaerobic digestion and turn municipal wastewater sludge into renewable energy

Discover how to best to increase sludge quality without incurring excess costs.

Anaerobic digestion is a critical element in a sludge management plan for wastewater treatment. Its role is to kill any pathogens in the raw sludge, produce a more desirable final product, and enhance the amount of biogas/energy generated by the process. The main issue is how best to improve the sludge quality without incurring excess costs.

The role of anaerobic digestion in water management

Treatment and disposal of sewage sludge accounts for between 25% and 65% of a wastewater treatment plant’s total operational costs. Leveraging the value inherent in the produced biogas can significantly increase a utility’s profit margin.

Sludge is a low-cost carbon resource that is normally spread on agricultural land as a fertilizer or sent to landfill. Both options are increasingly under pressure due to tighter regulation and increased costs. With further processing, sludge can be incorporated into construction materials or used as an incinerator fuel, but whether it finds uses at all very much depends on the sludge quality and pathogen reduction.

Anaerobic digestion plays a significant role in managing sludge because, in addition to killing pathogens, the organic matter represents a considerable energy resource that can be easily exploited, transported or used. Anaerobic digestion is normally a net energy provider, generating biogas that can be further processed into electricity. Both of these products can be used on the wastewater treatment works site or, increasingly, further afield.

How anaerobic digestion eliminates excess costs

The industry’s desire to leverage the value inherent in its sludge is driven by the need to manage OPEX costs (operational expenditure—maintenance, operation and repair). This focus has arisen because the major CAPEX (capital expenditure) spend to meet regulatory requirements is largely over but operating costs, key among which are electricity and maintenance, continue to rise.

Improving the quality of anaerobic digestion products helps to ensure that overall energy consumption is reduced and that the sludge digestion system is providing more energy than it consumes. Considerations include which parts of the treatment train are most likely to yield enhanced performance for the simplest, least costly input. This means looking for improvements in the earliest parts of the process.

Improving processes and performance

Grit removal, sludge screening and primary scrapers are key to improving biogas yield and improving process performance. If not effectively removed, grit not only takes up vital space in the digester but also causes significant wear on vital assets such as pumps. It also clogs pipes, channels, and downstream treatment systems — incurring expensive downtime and costs for repairs and for removing accumulated grit.

Improving primary tank scrapers can help to ensure that the maximum amount of carbon-rich primary sludge is sent to the digester. It has the additional benefit of reducing the requirement for aeration in secondary treatment processes. Sludge screening is important in situations where low-quality sludge from smaller treatment works may not have effective (or any) grit removal systems. Fine tuning the anaerobic digester performance is a vital part of making a wastewater treatment plant energy self-sufficient. It can even enable it to become a net energy exporter.

Achieving ROI as well as renewable energy

It is vital to keep the digesters working, ensuring as much sludge as possible enters the process so that anaerobic digestion can take place. The ROI lies in the increased biogas yield from securing a high-quality sludge, and ensuring that the digesters function at maximum efficiency by entraining the least possible grit.

So, when improving anaerobic digestion and seeking to turn sludge into a valuable energy source, it is vital to examine the efficiency of several key early processes:

  • Grit removal
  • Primary tank scraper performance
  • Sludge screening

If all these are working optimally, not only will sludge quality and biogas yield be significantly enhanced, there will be onward benefits to other downstream processes and significant reductions in the increasingly expensive electricity bill — potentially to the point of becoming an energy exporter.

Takeaways

Improving anaerobic digestion means examining efficiency of key early processes, including grit removal and sludge screening.

Anaerobic digestion can eliminate excess costs. The process ensures that the sludge digestion system is providing more energy than it consumes.

ROI lies in the increased biogas yield from securing a high-quality sludge and ensuring that the digesters function at maximum efficiency.

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