Sludge Treatment - Process intensification and optimisation
Aidan Cumiskey of Monsal examines opportunities for optimisation of existing sludge assets.
The big change that has occurred recently is the need to, not only process the sludge but to produce a product which will satisfy defined quality standards. The whole world must move in this direction and the legislative processes are slowly and remorselessly grinding away throughout the developed world. The UK will be required to meet the relevant EC directives and there are many that have a direct effect on sludge treatment and disposal matters including landfill, nitrates, incineration, soil and sludge recycling to land. The UK has also had to consider the widespread adoption of new sludge standards in order to satisfy the requirements of the farming and food production industries. In this the UK is following closely in the footsteps of the US, which through the application of the EPA Code of Regulations part 503 is proceeding in the same direction.
The challenge for the industry is to improve product quality without losing the increased throughputs which have been achieved in recent years. The improvements required are not just in terms of absolute product quality but also the day to day variations. These reflect not only variations within the processes themselves but also in the raw feed and to some extent, the ambient temperatures which have a significant effect on pathogen reduction rates.
Monetary pressures will mean that the industry is faced with doing more for less – process intensification and optimisation.
Researchers and manufacturers have responded to the challenge and technology development in sludge treatment is more active now than it has ever been. Much of the development is aimed at improving the anaerobic digestion process especially with regard to pathogen and solids destruction and energy production. Digestion has got to be better in the future than it has been in the past; and reliable, efficient and effective technology will be an absolute must. In general the approach adopted is to add additional unit processes to facilitate the improvements, e.g. pre-pasteurisation, Cambi thermal hydrolysis, pressure, mechanical and ultrasound disintegration for improving the treatability of waste activated sludge (WAS) solids. Other processes seek to optimise the potential of anaerobic digestion by phase separation, e.g. two phase processes and Enzymic Hydrolysis. Additional post digestion process such as chemical stabilisation have also found favour to provide pathogen reduction capability.
There are now a credible number of full scale advanced digestion facilities in the UK including Cambi thermal hydrolysis, phased digestion, thermal pasteurisation; and the numbers are growing. Many utilities are running pilot facilities as they seek to identify innovative solutions. This is good news for the industry as we will have access to these technologies in readiness for AMP4. In addition we can learn from the growing pool of knowledge in the US and mainland Europe where there are now many such plants which have been operational for a number of years. One such example in the UK includes United Utilities (UU) who have now implemented a full scale advanced digestion process and eliminated the need for pathogen kill tanks at one of their regional sludge centres (Figure 1).
This process has resulted from extensive research by UU into de-activation of pathogens over a number of years. Full scale performance has demonstrated that very good de-activation can be achieved at lower temperatures than previously anticipated. This process is now a viable alternative to pathogen kill tanks or other treatment technologies for securing the treated sludge standard on existing anaerobic digestion plants without compromising throughput.
The volume of WAS produced has significantly increased as a result of implementation of the UWWTD; which has increased the loading to existing sludge treatment plants. Furthermore the nature of WAS renders it less amenable to biological treatment. Pre-treatment is one way to counter this problem. Poor digestibility results in lower VS destruction and poor dewatering operations for water utilities. There has been a surge of interest in Ultrasound technology recently as a pre-treatment process. The technology is not new in mainland Europe and in relation to digester optimisation and energy production (CHP); ultrasound is claimed to increase gas yield by up to 50% on full scale facilities in Germany between 40,000 pop eq and 750,000 pop eq (Figure 2). This improvement in digestibility has been accompanied with an improvement in dewaterability of up to 7% points, there is however no improvement in pathogen reduction yet demonstrated.
Many sludge treatment plants are now being upgraded to handle larger throughputs; invariably meaning more thickening and de-watering operations on site. Traditionally liquors have delivered additional BOD loads of about 5-10% and higher ammonia loads of 20-30% to the main wastewater treatment stream. The increasing numbers of regional sludge centres where large volumes of sludges are imported to centralised more controlled facilities will place a greater strain on the wastewater stream with an increase in the ammonia load by as much as 50% now possible. The simplest process option for liquor treatment is co-treatment with the existing wastewater stream. However this is becoming more difficult and separate treatment of liquors is financially attractive on large centralised treatment plants (Figure 3).
The water industry has responded to the pressures in sludge treatment with innovative new unit processes. These are particularly suitable for large centralised sludge treatment plants and will allow compliance with new environmental requirements without sacrificing the gain made through process intensification. The sludge market is truly becoming more global and we now have a much greater portfolio of research, products and processes to deal with these pressures. The introduction of new technologies from other markets will place greater emphasis on sludge integration skills. They will be successfully integrated into our plants by more specialised teams in the future, where optimisation of existing assets will become part of the contract delivery strategy. There is little doubt that we will continue to respond successfully to the challenges ahead within our industry.
The author would like to thank Colin Brade (Monsal), Dr. Maxine Mayhew (UU) and Keith Panther (Dirk) for their valuable contributions in the preparation of this article.