A case for controlled composting
Windrow composting can be difficult to manage, but conditions in a closed system can be closely controlled. Sludge consultant Jim Walker makes a case for in-vessel composting.
Until a few years ago water companies automatically recycled sewage sludge to land; it was the obvious thing to do, it was reasonably cheap and they were being sustainable before sustainability became fashionable.
Then food safety organisations such as the British Retail Consortium (BRC) began to raise questions and the government produced the Safe Sludge Matrix, a document that has changed the perception of sludge disposal to land forever. Instead of it being a reasonably discreet activity shared by water companies, farmers and the occasional angry environmental health officer, it has become a fairly public issue.
After the introduction of the Matrix, water companies soon realised that the sludge would have to go somewhere else. Most of them began to install dewatering facilities so that they could at least make a cake to be stockpiled and as a last resort landfilled.
Then they began looking at what could be done if the agricultural route was lost. Those who had built incinerators could afford to relax but even they realised that getting permission to build more incinerators would get more difficult and expensive. A number of companies saw sludge drying as the answer, it had been pioneered by Wessex Water at Avonmouth, and several more dryers were constructed. There was the question of whether to dry raw sludge or digested sludge and examples of both could be found, sometimes within the same company. Drying produced a low-volume product which didn't look like sewage, could be used in agriculture and as a last resort would be cheaper to landfill than liquid sludge.
Dried sludge also offered the possibility of use as a fuel, thus getting away from agriculture altogether. This has taken a step towards fruition with the issue of a change of authorisation to allow burning of dried sewage sludge at Longannet power station in Scotland. The use of dried sludge as a fuel however seems to be dependent on having large quantities of pellets available to satisfy the needs of the end-user.
Another high-tech answer for smaller sites, namely gasification and energy generation, has the added advantage that the whole system is under the control of the water company and is therefore farmer-proof, weather-proof and supermarket proof. But this is also expensive.
Then came AMP3 and following Ofwat's price determinations all the companies found themselves with less money to spend. Some of the first casualties have been dryer programmes while companies look to see if there is anything cheaper.
Windrow composting is cheap, but it does have disadvantages such as odour, if windrows go anaerobic. The second problem is the Matrix, which precipitated the upheaval in the first place. It is quite difficult to reliably audit windrow composting to show if it is actually producing an 'enhanced treated' product, i.e one with very low pathogen counts.
Under the Matrix "conventionally treated" biosolids can be used for food crops, however at some point in the future only "enhanced treated" products will be allowed. Therefore any new sludge treatment facility must have the capability of carrying out enhanced treatment or must at least be capable of being extended to do so.
In-vessel composting has several advantages over other systems. For instance, each batch can be monitored to guarantee enhanced treatment. Odours are contained and can be dealt with, and because there is no exposure to rain or wind, temperatures and moisture contents can be controlled to give a faster composting process. The disadvantage is that the increased complexity of in-vessel systems pushes prices up but they are still cheaper than sludge dryers.
The Composting Association's directory of in-vessel composting systems lists 25 different systems in five categories. At the present time there are at least nine companies selling into the UK market with systems ranging from cages and towers to tunnels and agitated bays. There is no doubt in-vessel composting can be less costly than other types of treatment. Only lime stabilisation costs less.
In-vessel composting can produce a product that the BRC will accept. The remaining question is what to do with the compost if the agricultural route is lost for another reason. Farmers could have insufficient confidence in the product and other sources of nutrients might be easier and cheaper to obtain. In the short term, non-agricultural uses such as landscaping and rec-lamation are still an option. But the government's strategy on waste disposal needs to be implemented quickly.
Speaking at the recent Sewage Sludge and Treatment Conference, Tricia Marcuse of the Babtie Group warned delegates: "There is soon going to be a lot of compost about. And I do not believe there will be sufficient market for all this compost if the agricultural market is lost."
On the positive side it seems Ofwat has assumed conventionally treated biosolids will have access to agricultural land in AMP3 and I would expect the government to support this approach. Both the UK government and the EC are clear that they want to see the agricultural route continue, provided safety standards are met.