Sewage works - with a bit of composting

Whither the sludge left over from wastewater treatment and dewatering applications? An Ipswich, UK, business promotes composting as a more environmental, sustainable solution.

In the quest to “clean up our act” in modern society, urban sewage and the way its disposal is managed has become an important issue. John Jardine, managing director of Covered Systems Ltd., a man who knows a fair bit about traditional aerobic aeration, explains why he thinks composting might be one possible way forward for sewerage sludge.

Referring to a March 2003 article in the New Scientist, which discusses benefits of sewerage composting, Jardine mentions Michael Rouse, past UK chief drinking water inspector, who was quoted as saying that if Britain were planning sewage disposal from scratch today, “we wouldn’t flush it away - we would collect the solids and compost it”.

Municipalities today are seeking better methods to dispose of ever-increasing amounts of sewage sludge. Composting offers a double-barreled solution to this perceived problem. It not only disposes of sludge but also converts it into a product that’s more aesthetically acceptable, safer from a health standpoint and useful as a soil additive beneficial to the growth of plants. Composting is a cost-effective, environmentally acceptable alternative to such ultimate disposal methods as incineration, ocean dumping and land-filling.

The general climate for sludge composting is healthy. For example, more cities in North America are turning to composting due to public perception of recycling. As it gets increasingly difficult to site combustion facilities; and where land application isn’t feasible, composting is becoming the preferred method for handling sewage sludge.

Animal By-Product Regulation

Returning from a recent visit to Countrystyle Composting’s site near Sittingbourne, where Covered Systems is supplying a 30,000 tonne per annum in-vessel facility, Jardine’s mind turned to the idea of how his patented aeration system could be used to help other composting materials apart from organic domestic and commercial material.

“We have been selected by our customers from a number of IVC suppliers due to our unique processing to the EU ABPR standard, low operating costs, low capital cost and our wealth of operational experience of operating our own IVC composting facilities, and also for the use of aeration during maturation”, says Jardine, “so our experience stands us in very good stead for looking at other commercial activities”.

Good composting depends on many things that have a direct and indirect influence on microorganism activity; these include the type of raw material being composted, moisture content, temperature and aeration.

These fundamental ingredients are enhanced using the Mistral system, which eliminates normal problems if using in-ground or aboveground compost aeration systems. The design increases air circulation, speeding up the composting process and eliminating odours.

When used on outside windrow composting sites, the system negates the need to turn the windrows during maturation, which is when many problems occur associated with potentially unpleasant odours arising from anaerobic parts of the windrow. It eliminates these environmental problems and saves the operator cost of turning the compost by machine. Initial findings during a trial composting sewage with Mistral have been encouraging.

One Way Forward

Some obvious questions arise. Is sewerage composting safe and, if so, what do we do with the end product? There are some clear benefits of composting sewerage, the most obvious one being that it kills human pathogens and parasites, which makes compost a much cleaner material than the digested bio-solids.

In a historical survey, two scientists at Sydney’s Agriculture’s Biological & Chemical Research Institute, intrigued about the possibilities using composted sludge in vegetable growing, undertook a study that lasted several years. The vegetables used in the trials were lettuce, beetroot and aubergines, representing a leaf, root and fruit crop. As a comparison, control plants were also grown in untreated soil, and fed with a standard rate of inorganic NPK fertiliser. In every experiment, the crops fed on composted sewerage waste faired better and grew faster than those fed with the inorganic fertiliser.

Of course there are potential problems; one common to most composted materials is a temporary lock-up of nutrients, particularly nitrogen, in the soil. This ‘nitrogen immobilisation’, and occurs when the micro-organisms the compost require some nutrients, particularly nitrogen, and so they actually compete with the crop for nitrogen.

Heavy metal contamination is another concern and the trials identified the most problematic metals to be copper, zinc and cadmium. The experiments indicated that plant health was not in jeopardy, and the results were also highly favourable for human health, with no excessive heavy metal levels found in the edible portions of any of the crops tested.

Conclusion

Jardine knows this article won’t create the solution but hopes, by talking more about sewerage composting, we might be able to create more of a debate about what we can do with a commodity most of us would prefer to forget.

For more on Covered Systems Ltd., visit www.coveredsystems.co.uk