Slashing costs not standards
Pressures on sludge disposal routes have given water companies cause for concern and led to UK trials of a system which tackles the problem at source, in the activated sludge process
The activated sludge process is highly effective at treating wastewater but normally produces large volumes of sludge residues. Finding economic and environmentally acceptable means of sludge disposal, however, is a major and growing challenge. The options are reducing all the time - disposal of sludge to sea is no longer allowed, landfill sites are finite, which leaves incineration or spreading sludge on land as a fertiliser as the most commonly employed alternatives.
Incineration, although effective in reducing the volume of sludge, is expensive both in terms of the initial investment and operating costs. Planning restrictions reflecting environmental and public concerns tend to have made this a less-used option, with opponents arguing the process generates large amounts of CO2 and other greenhouse gases, which contribute to global warming, as well as other harmful atmospheric emissions. Agricultural utilisation, although supported by the UK government as the BPEO, is limited by land availability, spreading rates to meet crop requirements and to control the build-up of potentially toxic elements in the soil and by competition from other materials. The public also tends to have a negative view of using sewage sludge as an agricultural fertiliser.
These concerns have been largely addressed by the Safe Sludge Matrix, produced by the farming, food and water industries. This agreement addresses sludge treatment requirements and also details which crops can receive what type of sludge. Despite the problems for many in the water industry the use of treated sludge in agriculture remains the most economic and practical means of disposal.
For the water companies, however, disposal to land leaves much to consider - one of the main issues being the cost of handling large volumes of sludge. Treatment processes that reduce pathogens in the sludge can only take so much. Transportation and handling costs are incurred, both to tanker wet sludge from WwTWs to sludge treatment centres and for taking the treated sludge to farms.
Removal of water by thickening, dewatering and drying will reduce transportation costs, at a price, but the remaining sludge quantities are still large. Costs to the farmers remain low, so generally the increased costs have to be absorbed by the water companies.
It is clear there could be an economic case for a process that could effectively reduce sludge generation without detriment to the environment or sewage treatment efficiency. This is why Degrémont is trying to tackle the problem at its root, in the activated sludge plant. Using either the company's Biolysis O or E systems the amount of sludge produced can be cut significantly.
Biolysis works by controlling the behaviour and numbers of bacteria that form the basis of the activated sludge process. The two processes 'stress' the bacteria in different ways, killing and solubilising some and stressing those that survive so they consume energy in recovery rather than reproduction. This change in behaviour reduces the generation of excess biological sludge. Although the systems induce stress in the bacteria by different means both limit the production of filamentous bacteria, which cause sludge bulking, and considerably improve sludge settlement. In Biolysis E the process is a biological one, a heat-activated natural enzyme is used to affect the bacteria. The system was originally developed by a Japanese company with which Degrémont signed a partnership deal. In Biolysis E an in-line reactor operates on a loop, abstracting and returning effluent to the activated sludge tank and generating the aerobic and thermophilic conditions that stimulate development of a particular type of microbe, which is inert at normal room temperature. When activated, the microbes produce enzymes that attack the outer membranes of the bacteria present in the sludge, reducing their ability to reproduce. Biolysis O technology depends on the addition of ozone and is the result of work at CERDEG, Degrémont's research centre at Croissy, France, which began in 1997. In the Biolysis O process, liquor extracted from the activated sludge basin is contacted with ozone in a reactor and returned to the activated sludge tank. When the bacteria come into contact with the ozone it causes oxidisation, which stresses them sufficiently to destroy some and again ensure others do not reproduce. The choice of system is mainly dependent on the treatment processes already in place - a plant with a heat source from, say, a digester, might opt for Biolysis E. Both systems are able to reduce the volumes of sludge generated by a large margin.
A demonstration plant for Biolysis O in northern France consistently produced sludge reductions of between 30-80%. Several full-scale works in France now have these processes up and running. A price comparison of treatment systems with and without Biolysis, based on French prices, showed no increase in the total cost - any small increases in installation and operation costs for the Biolysis equipment were more than offset by economies in other areas, particularly sludge disposal and digestion.
This is a view that is now being tested in the UK at a Northumbrian Water WwTW, Broomhaugh, near Hexham. The works has a 7,500PE, almost entirely domestic, and serves the communities of Corbridge, Stocksfield and Riding Mill. Treated effluent discharges to the River Tyne.
There are four partners in the trial - Northumbrian Water, Severn Trent Water, Yorkshire Water and Degrémont. The trials arose after the Biolysis process was first launched in early 2002.
Degrémont held discussions with Northumbrian in early 2003, when the idea of a pilot trial at a UK site was first mentioned. Degrémont's technical manager Stewart Bell said: "From the water companies' point of view, the objective is to evaluate sludge minimisation as a radical alternative to conventional sludge generation, treatment and re-use or disposal. From Degrémont's point of view, the company is keen to demonstrate this emerging technology to a wider market, including the UK. It also wishes to address the natural concerns of the water companies on adopting a very different approach to managing sludge."
After initial discussions, Degrémont and Northumbrian decided to look for other partners to widen the involvement in the project. Severn Trent and Yorkshire, both of which have a similar interest in sludge minimisation, agreed to join in a collaborative trial. The pilot plant was delivered to site just before Christmas last year.
John Churchley of Severn Trent commented: "The waste hierarchy places elimination and minimisation of waste at the top and hence should be the preferred strategies. Sewage treatment processes have complex interactions with the environment, however, and all of these interactions must be studied in evaluating any different approach. This trial is designed to enable the water companies to have the required information to determine whether sludge minimisation by Biolysis is economic, reliable, operable and has acceptable impact on the sewage treatment process." Because Biolysis reduces the mass of surplus activated sludge, it increases the primary/secondary ratio. This has the double benefit of reducing the total volume of sludge to be treated and recycled and improves the treatability of that sludge - primary sludge can be both digested and incinerated more easily than secondary.
Commissioning was completed during the spring, while the existing plant performance was monitored and analysed in detail to ensure there is a comprehensive overview of the existing system before Biolysis starts. The Biolysis plant at Broomhaugh consists of three modules - one houses the mixer and control panel, a second the feed tank, and the third holds the ozone generation equipment. The feed tank module sits on top of the mixer module and the trial unit is placed adjacent to the aeration lane. The Biolysis plant receives return activated sludge, which comes into contact with ozone generated from pure oxygen, before returning to the aeration basin.
The current treatment system at Broomhaugh comprises two primary tanks, two aeration lanes and two final settlement tanks. The works average daily flow is 4,000m3/d. The Biolysis plant has been installed on one of the aeration lanes, treating 50% of the works' flow. The works was chosen for the trial because it was about the right size for the pilot trial unit available (3,500PE) and, by applying Biolysis O to one entire lane, a side-by-side trial could be carried out. A full physical separation of the lanes and return activated sludge streams has been effected for the trial. Other factors in the choice were that the works was performing well, thus a trial should not risk compliance and, importantly, there is aeration capacity available to treat the COD solubilised by Biolysis. Two phases of trial are envisaged - one with a targeted sludge reduction of 60% and a second with a targeted reduction of 80% or better.
Each phase will last six weeks, sufficient for reliable data to be generated to gauge the reduction of the plant's biological sludge production. There will also be two seven-day intensive monitoring periods in each phase of the trial to investigate the wider impacts and monitor operational performance. Tom Taylor of Yorkshire said: "Getting a robust trial set up has taken a considerable time. It has been important to carry out detailed characterisation of the activated sludge behaviour prior to the application of Biolysis. Preparing the works for this and monitoring over appropriate reference periods has taken three months. At the end of April, the first Biolysis phase began."
Northumbrian's technical strategy manager for sludge Mike Rewcastle said: "Since the proposed works was confirmed as acceptable to all, there have been many challenges to overcome in getting the trial set up. There have been the usual logistical things such as lifting and moving, transport and co-ordination of setting up a trial on a 'live' works - not to mention winter conditions. NWL has had to modify the works to totally separate the two lanes - this involves an over-pumping arrangement for RAS - monitored by telemetry to protect the works' compliance.
"Designing the monitoring programme to generate reliable valid data also presents a challenge. Local operators need to be involved in the trial to retain control of works' performance and play their role in the trial process. These challenges have been largely overcome but all will continue as aspects to be managed during the trial."
Rewcastle added: "We want to answer the question 'should sludge minimisation play a role in our future sludge management strategy?' The NWL strategy has already moved sludge management from mainly disposal (landfill and sea disposal) to re-use of treated sludge products as an agricultural fertiliser and as an alternative fuel in cement manufacture. While this transformation was prompted by legislative changes, we feel it is the right time, as responsible companies, to look at this new development as a potential to take another step up the waste hierarchy, if it is considered to be the right choice for the business, on behalf of its customers and the environment.
"Minimisation of biological sludge at some of the largest activated sludge works could allow more efficient use of existing sludge treatment facilities and could postpone or reduce the need for some of the future investment in new sludge treatment facilities. This trial should generate information to assist in making these crucial future decisions."