Still seeking the biosolids glass slipper? Twenty Years of Progress in the Treatment & Disposal of Biosolids

By Paul Lowe (Founder European Biosolids & Organic Residuals Conference) and Alan Whipps, Pell Frischmann

The treatment and disposal of sewage sludge was for many years the Cinderella of the sewage treatment process. The emphasis of sewage treatment was achieving effluent quality standards with the aim of cleaning up the watercourses into which the effluent discharged. However, meeting improved effluent quality standards required increased treatment capacity which in turn generated greater sludge production; in many cases this posed a growing problem for those responsible for the treatment process. For large conurbations sea disposal was a simple solution. For some the solution was to dump the sludge into large lagoons in the hope that the problem would go away, thankfully others took the problem more seriously and invested in anaerobic digestion followed by recycling to agricultural land. In Europe the latter was brought into sharp focus with the publication and enforcement by national governments of the EC Directive 86/278/EEC of 12 June 1986 on the protection of the environment, and in particular of the soil, when sludge is used in agriculture. Thus the emphasis moved towards developing sustainable solutions for the treatment and recycling of sewage sludge, particularly as the sea disposal route was terminated in 1998.

The US was acutely aware of the growing problem of public acceptability of sewage sludge as a valuable resource. In order to overcome this issue they introduced a change in emphasis by introducing the term Biosolids. The US Environmental Protection Agency (EPA) was required under the Clean Water Act Amendments of 1987, to “develop a new regulation to protect public health and the environment form any reasonably anticipated adverse effects of certain pollutants that might be present in sewage sludge.” This led to the publication of the EPA Guide to Part 503 rule September 1994 introducing a two tier quality definition for Biosolids Class A and Class B.

The European Biosolids and Organic Residuals Conference

The US Water and Environmental Federation (WEF) realised there was a need to support the development of science and technology of sewage sludge and established an annual conference devoted to this development. In a similar way the needs of the UK were met through the establishment in 1995 of the “European Biosolids & Organic Resources Conference”, intended to provide a discussion forum for industry practitioners. This first conference included papers on: composting, lime treatment, thermal drying, pelletisation, gasification, ultrasound treatment, marketing, recycling and disposal. Over the next nineteen years over 1,500 papers would be presented marking the trends in regulations, sustainability, research and process technology.  This article reviews these trends and uses this to predict where the biosolids industry might be heading over the next decade.

20 years of Process Trends

In preparation for the loss of the sea disposal route (1998) significant investment was made in sludge incineration for larger conurbations. Revived interest had been generated by the building and commissioning of the fluidised bed systems by Yorkshire Water replacing the multi-hearth processes of the past decade. What was novel about the new generation of incinerators was the emphasis on achieving the auto-thermal conditions and control over the stack emissions. By installing drying plants or advanced dewatering systems upstream of the incinerator feed, the incinerators could be run without support fuel. The introduction of emission abatement technology and re-heating of the emission gasses prior to discharge resulted in an invisible plume, except at very low winter temperatures. However, concerns over dioxins was to result in raising the furnace discharge temperatures from 850 OC to over 1,200 OC. In a number of cases this higher operating temperature reintroduced the need for a fuel supplement in the afterburners. So with increasing operating costs the interest in incineration began to decline in favour of alternative solutions.

Another rising star of the early 1990’s was the introduction of thermal drying technology encouraged by the installation of the Swiss Combi process by Wessex Water, installed specifically to replace the sea disposal route and exploit agricultural recycling.  This installation was followed by a Buss thermal drying plant at South West Water. The market potential of thermally dried biosolids’ pellets or granules was significant and offered the opportunity to influence how the agricultural market would develop. The UK’s largest drying plant was built by Northumbrian Water at Bran Sands. However, as more plants were installed, there were concerns about the explosive hazards of the drying system and problems associated with dust. The significant increase in power and gas prices together with safety upgrade requirements led inevitability to plant mothballing and closures.

Thickening and dewatering is a constant theme in sludge handling with competition for the dewatering market vying between three systems. Plate press technology saw the introduction of the membrane rubber 2m x 2m filter plate that promised high dry-solids dewatering. High solids centrifuges were developed and introduced and two stage belt dewatering systems emerged. Of significance much of the high solids success came from the development of polyelectrolytes with greater control over their addition and mixing to avoid over or under-dosing. In recent years new systems have continued to enter the market, notably the Bucher Press attracting a great deal of interest.

One technology that emerged at the end of the 1990’s was the thermal hydrolysis process developed by Cambi. This process uses a high pressure/high temperature system to breakdown the structure of the untreated sludge. The concept was not new as heat treatment had been around in the late 1960’s and early 1970’s with the Farrar pipe reactor and the Porteus batch reactor. Both systems were effective means of conditioning the sludge prior to dewatering. The main problem with the Farrar reactor in particular, was the release of strong unacceptable odours and also a biological overload of the sewage treatment works treating the dewatered liquors generated by the dewatering plant. This could result in a subsequent failure of the works to meet the discharge consent. The genius of the Cambi engineers was to pass the heat-treated sludge to anaerobic digestion. This not only improved the solids content of the digester but eliminated the two basic problems experienced by previous heat treatment processes. Other hydrolysis processes are now available such as the Veolia Biothelys, and their effectiveness has been fully debated during past conferences, giving process designers other options to consider.

The workhorse of the sludge treatment process for many years has been the anaerobic digestion process. This process has been around for almost a century but for many years it relied on long retention periods (over 35 day) and lower than optimum temperature. Noone and Brade made a significant change when they introduced the low-cost, glass coated steel reactor making the process suitable for small works. However, the impact of the thermal hydrolysis process resulted in a higher solids concentration in the reactor, with lower retention time and this has made the digestion process more effective. Process trains involving mesophilic, thermophilic anaerobic and aerobic digestion combinations have all been trialled and these offer some innovative alternatives in specialised niches. What has to be remembered about the anaerobic digestion process is that over 35% of the solids are converted into biogas thus releasing a potential energy income for the operator. In recent years techniques to maximise the gas yield have been developed and incorporated into the process design. In addition research into effective digester mixing has demonstrated this to be a key requirement to maximise the process efficiency.

Other technologies have had their moment of glory but have not quite made it into accepted practice. Composting is one such process, which from time to time has appeared in the proceedings but has remained outside the mainstream of process solutions. The use of reed beds also fills a particular niche as a simple solution option for sludge dewatering, whilst vermiculture, although appearing to provide a “green solution”, has come and gone. Odour issues have remained on the agenda and solution options have been described however, as housing developments continue to encroach on sewage works boundaries, investment in cover and contain solutions are now routinely implemented.

Recycling and Disposal

Of course the solution to long-term sustainability can only be realised once the treated biosolids leave the sewage works gates carrying with it the benefits of plant nutrients and rich soil conditioning organic matter. There has been a wealth of research on the scientific approach to land application over the past 19 years and our understanding of control of pathogens, metals, the correct application of nutrients, the cost of transport and spreading  has meant we can recycle this material safely and with confidence. The activity of the British Retail Consortium (BRS) and potential of loss of the agricultural outlet resulted in the ADAS Safe Sludge Matrix and establishment of two standards for treated biosolids: Conventional Treated and Enhanced Treated.  To provide quality assurance and control HACCP principles were introduced for sludge treatment processes, thereby securing the agricultural outlet. The Landfill Directive also played a pivotal role in removing an outlet for biosolids and the introduction of Renewable Obligation Certificates (ROC) stimulated renewed interest in anaerobic digestion and supported the installation of thermal hydrolysis processes.

The Next Decade

What of the future? Because of its origin the treatment and disposal of Biosolids will always remain an issue. Waiting in the wings is the European Commission’s Working Document on Sludge, first drafted in April 2000, which sets out possible revised definitions for sludge and sludge processes. Once the economic situation improves for the European nations it is likely that this document will be dusted down and revisited.

Emphases on sustainable energy solutions are likely to drive the industry towards even greater organic and volatile matter destruction by anaerobic digestion. This should drive forward emerging technology solutions and interesting solutions probably based on combinations of existing technologies.

The farming industry around Europe cannot stand still and modern farming machines are entering the market, enabling the precise sowing of individual seeds to give maximum yields. This could drive the biosolids industry into developing products and machinery, which compliment that precision application of nutrients and organic matter.

The potential to derive more valuable organic materials from the digestion process whilst recovering the important nutrients N and P mean that interest in Biorefineries will continue to grow, and the first results of UK research into the application of dewatering liquor as an algal growth medium are starting to appear.

What is clear is that the treatment of sewage sludge and the production of biosolids products will continue to challenge the engineers and scientists of the next decade. Their concepts and solutions will continue to make the European Biosolids and Organic Resources one of the most important conferences in the annual calendar.

Paul Lowe and Alan Whipps will be speaking on this theme in more detail at Aqua Enviro’s 20th European Biosolids & Organic Resources Conference & Exhibition 9th-11th November 2015.  Please see the website for more detail



Posted 15th September 2015
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