Composting - a cleaner disposal for sludge

In the quest to "clean up our act" in modern society, urban sewage and the way we manage its disposal, has become an important issue. John Jardine, who knows a lot about traditional aerobic aeration, explains why he thinks composting might be one possible way forward for sewerage sludge.

In an article in the New Scientist which talked about the benefits of sewerage composting Michael Rouse, formerly the UK's chief drinking water inspector, 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".

A key issue today is that municipalities are seeking better methods to dispose of ever-increasing amounts of sewage sludge. Composting offers a double-barrelled solution to this perceived problem.

It not only disposes of sludge but also converts it into a product that is more aesthetically acceptable, safer from a health standpoint, and useful in many important practical applications as a soil amendment beneficial to the growth of plants.

Composting is a cost-effective and 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, in North America more cities are turning to composting because of the public's perception of recycling.

As it gets increasingly difficult to site combustion facilities, and where land application is not feasible, composting is becoming the preferred method for handling sewage sludge.

From as early as 1970, American technicians were researching sludge composting. A very detailed study at the Blue Plains Wastewater Treatment Plant in Washington DC2, completed in 1980 looked at the feasibility of using the aerobic process.

Here, the plant - it was serving seven authorities - was producing about 300 wet tons (23% solids) of digested sludge daily.

Interestingly, even back then, disposal of the sludge by releasing it into the nearby Potomac River or barging it for off-shore dumping into the Atlantic Ocean were considered environmentally unacceptable.

Consultants recommended sludge incineration as the best disposal system. However, until the incinerators could be constructed, the Maryland Environmental Service, assumed the responsibility for disposing of the sludge by land application.

Several methods were investigated, including trenching and land spreading.

Technicians also began research on composting of sewage sludge.

A successful windrow method using woodchips as a bulking and moisture-absorbing agent was developed for composting digested sludge.

During 1973 and 1974, the Beltsville facility windrow composted in excess of 50 wet tons of digested sludge each day.

By 1975, the Blue Plains Wastewater Treatment Plant had increased its capability for removal of solids from the wastewater.

This resulted in the production of an additional 200 wet tons of undigested or raw sludge (a mixture of primary and activated secondary sludge), for a total output of about 500 wet tons of sludge per day.

The finished material, demonstrated worth as a soil amendment stimulant to plant growth. The study concluded by saying that if relatively simple control procedures were followed, the compost would become free of primary human pathogens because of the lethal effect of heat generated during the composting process on such organisms.

Eliminating odours
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, I thought about how my patented aeration system could be used to compost a range of organic material.

We have been selected by our customers from several IVC suppliers because of:
  • Our processing to the EU ABPR standard
  • Low operating costs
  • Low capital cost
  • Our wealth of operational experience of operating our own IVC composting facilities
  • For the use of aeration during maturation
Good composting depends on many things that have a direct and indirect influence on the activities of the micro-organisms; these include the type of raw material being composted, moisture content, temperature and aeration.

These fundamental ingredients are enhanced using the Mistral system, as it eliminates the normal problems if using in ground or above ground compost aeration systems. A simple yet innovative design increases air circulation, speeding up the composting process and eliminating odours.

Maturation phase
When used on outside windrow composting sites the system negates the need to turn the windrows during maturation, turning is when many of the problems associated with potentially unpleasant odours arising from anaerobic parts of the windrow occur. The odour problem arises during the maturation phase of the composting process and once the bulking agent has been removed.

The Mistral system eliminates many of these environmental problems and saves the operator cost of turning the compost by machine.

Although this is a potentially new application for Covered Systems, the principles are the same for a company that recently won The Composting Association's Innovation in Composting Technology & Machinery Award for its Mistral system.

In ground or above ground systems are very expensive to install, limit the use of the concrete curing areas, create leachate collection problems and need constant cleaning.

Also normally the aeration is positive, in other words it is forced up through the compost and therefore makes the collection of the exhausted air very difficult unless it is inside a building.

The Mistral system overcomes these problems with a series of vertical pipes that are inserted mechanically into the compost from the top once the windrow or pile is formed or during formation.

The vertical pipes are connected to a larger central air pipe that carries the air from the bottom of the windrow back to the fan, which is sucking the air from the windrow.

Air is then collected via this exit air pipe and then discharged to atmosphere via a bio-filter. The system can be used on any existing surface, is simple, low cost and contains any leachate within the compost mass where it is needed.

This negative aeration technique is being used by Covered Systems in their in-vessel installations and has proved itself to be an efficient method of compost aeration.

There is greater moisture and heat retention in the compost pile leading to faster degradation of the compost and a much more homogeneous mass.

Each windrow has its own fan and timing mechanism that is programmed to complete a pre-determined aeration cycle, constantly, 24 hours a day.

The system has already been proven with a number of different feed-stocks from ordinary green waste, kerbside catering waste and vegetable waste.

The cost of installing the system and its ongoing use is around 70p per tonnage input of the site, including capital write off, over a five-year period and the labour needed to install the system.

Compost benefits
Some obvious questions arise. Is sewerage composting safe and if it is, 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 an historical survey, scientists at Sydney's Agriculture's Biological and 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. It is known as 'nitrogen immobilisation', and occurs when the micro-organisms within 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.

To conclude, by addressing the issue of sewage composting we might be able to create more of a debate about what we can do with a commodity that most of us would prefer to forget about.

John Jardine is managing director of Covered Systems.
T: 01449 721299.

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