Plenty of muck ­ but is there enough brass?

Safety and cost-minimisation were central issues at this year’s IBC conference on Sewage Sludge; Disposal, Treatment and Use. Peter Minting reports.

Since the banning of untreated sludge disposal at sea in 1998, water companies have been searching for the Holy Grail of sludge treatment processes, i.e one which satisfies concerns about public safety and results in a cheap, saleable product.

Despite this dilemma with regard to finding an ‘acceptable’ disposal route, a number of speakers at the conference were optimistic about their ability to provide safe and cost-effective treatment systems.

Co-combustion
Dr William Barber of the Dirk Group presented the results of a project to burn sewage sludge with coal at power stations. Dirk has developed a process where wet sludges can be dried out using the heat from the coal mills of existing power stations, as long as the power station is of a suitable design.

According to managing director Georg Dirk: “We narrowly missed out on an opportunity to install the system at a [west] Scottish power station. They said that if they had known about the system, they probably would have chosen it, but unfortunately they were already too far along with installing a sludge dryer plant. However, we think there will be other opportunities.”

Dirk has proven the effectiveness of the system at EnBW power station in Heilbronn, Germany. Here energy is recovered from partially dewatered sludge cake with a dry solids content of 25%.

Provided the coal used is of a relatively low moisture content, the burning process is not affected by the addition of sludge, which has a similar calorific value to low-quality brown coal. Raw primary sewage sludge typically has a calorific value of 25,000kJ/kg, and anaerobically digested sludge around 11,000kJ/kg.

Co-combustion has a number of advantages, apart from avoiding the need for dryers, including virtually no effect on pollution control costs at power stations which already have air scrubbers, and the opportunity to dispose of raw sludge.

Disadvantages may include the need to use coal of a low moisture content and the diff-iculty of persuading power stations to install the process in the first place. This may be less difficult if the station is owned by a combined energy and water utility. The ratio of sludge to coal is also quite low; at Heilbronn, the sludge accounts for just 4% of the fuel.

ATAD too slow
North West Water¹s biosolids product team, led by W.J Davies, has looked at the potential for using thermophilic aerobic digestion (TAD), autothermal thermophilic aerobic digestion (ATAD) or thermal hydrolysis at North West¹s Ellesmere Port STW. North West hopes to sell the digested sludge for use in agriculture, so it needs to be sterilised in order to meet the government¹s strict sludge matrix legislation (1998).

Mr Davies said: “TAD, ATAD and thermal hydrolysis were examined as potential retrofit options for the works. It was clear from the estimates that TAD was the best option on all three measures (capital expenditure, operating expend-iture and NPV). However, it is worth noting that the high capital cost of the thermal hydrolysis options was accounted for by the requirement for dewatering, water treatment and steam generation, which would suggest thermal hydrolysis would be more economical on a larger scale. ATAD proved to be the least favourable retrofit option, apparently due to the fact that it has a much higher hydraulic retention time in comparison with TAD, and would therefore need a larger number of reactors.”

Although it is possible to create an autothermal system which pasteurises sludge using the heat generated by aerobic digestion, (WWT September ‘99, p24) according to Mr Davies this can be tricky: “Using pure oxygen [to aerate the sludge] an autothermic condition can be achieved with just 24hr hydraulic retention time (Trim, 1984). However, in order to achieve stabilisation a minimum reactor retention time of seven to eight days is required (Pitt and Ekama, 1995). The feed sludge must also be sufficiently concentrated so that the ATAD does not become substrate limited, with a reduced heat output. Because of the difficulty in ensuring a consistent sludge quality very few ATAD systems have been in operation until recently.”

This is not such bad news for those equipment suppliers which have thrown their weight behind ATAD systems, as these can easily be modified to make use of an additional heat source. Instead, it becomes clear at this point why water companies have adopted so many different systems; in some areas the sludge quality may or may not lend itself to ATAD, and at larger plants than Ellesmere Port, thermal hydrolysis might be a better option than TAD.

Steam cleaning
Thermal hydrolysis was the process of choice for Thames Water’s Chertsey STW, as described by Peter McLoughlin, general manager of Simon-Hartley Cambi. The thermal hydrolysis plant was designed to process sludge from Chertsey and STWs at Chobham, Weybridge, Lightwater, Esher and Leatherhead. By far the greatest volume is supplied by the latter two STWs, in the form of raw, untreated sludge.

The system therefore needed to be extremely effective at pathogen destruction, because at Chertsey (like North West¹s Ellesmere Port STW) Thames has opted to try and dispose of the waste to agricultural land. The process chosen also had to be compact, as space was limited. The need for planning permission was avoided because the scale of the install-ation meant that it could be classed as a “maintenance area” and not a new building.

The sludges from the various sources are first mixed by a submersible mixer in a large 1500m holding tank. The blended sludge from the holding tank is transferred to duty/standby belt press units for dewatering. A macerator unit conditions the sludge before dewatering in Klampress units. Dewatered sludge cake is then transferred to a cake silo using positive displacement pumps.

The cake is pulped in a pulper vessel and pumped into two 10m3 reactor vessels. Here steam is injected into the pulped sludge. A temperature of 1750C is maintained for 30mins inside the vessels, at a pressure of 8.5bar. This is said to be sufficient to kill all pathogens, i.e producing a class-A sludge product which can be used on any type of agricultural land.

Steam and sludge from the high-pressure tanks passes into a reduced-pressure ‘flash’ tank. Excess steam is recirculated into the pulper vessel to maximise energy efficiency.

Mr McLoughlin advised potential users of the process “not to take any chances” with the odour control systems or ancillary instrumentation, as “thermal process plants can’t have days off.” A biological odour filter containing woodchips proved inadequate for the high-strength odours generated by the hydrolysis and a different system had to be brought in for this stage of the process.

  • Far-reaching effects of the matrix
    Since introduction of the ‘sludge matrix’ legislation in 1998, treatment standards in the UK have become increasingly strict. From December 31 2000, use of untreated sludge on land will be completely banned. Brian Chambers, principal research scientist at ADAS said: “Since 1998 there has been a marked increase in the production of lime stabilised, thermally dried and composted materials.” He added: “In many situations, a financial value is being realised for the products.” However, it is unlikely that farmers will pay for sludge products unless they are of a high quality. Sewage sludge only accounts for 2% of organic waste spread on farmland, compared with 4% for industrial sludge and 94% for farm manures.

    To ensure it is possible to find a market for processed sludge it is essential the treatment technology keeps pace with increasingly strict quality standards. The UK government is concerned about EU plans to reduce the concentrations of heavy metals (90% of samples) in sludge spread on land. The EU now plans to review the levels permitted every six years and if water companies cannot find a cost-effective way to remove the metals, this could effectively limit the amount of sludge used on land.



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