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The worm's turn

Water Sustainability

Wastewater companies and the food industry alike could benefit from a new, sustainable form of sewage sludge treatment that uses one of nature's most abundant creatures - the worm.

Hundreds of small rural treatment works operated by UK water companies spread their raw sludge directly onto farmer's fields, because it provides a cheap means of sludge disposal and a nutrient boost to agricultural land.

However, this seemingly happy situation is about to change, due to new European sludge disposal laws - currently being transposed into UK legislation - that will outlaw raw sludge disposal to land. This is because Europe wants to take better control over exactly which nutrients are being put into the soil and to prevent diffuse pollution into the many watercourses that meander between the UK's fields.

Instead, only fully treated sludge will be allowed to be discharged to agriculture.

For rural wastewater treatment works owners however, this poses a big problem. Providing complete sludge treatment for works so tiny they serve no more than 800 people would not be cost effective. So now water companies are faced with a challenge to find new, previously untried solutions to solve the small treatment works sludge problem.

One answer is being developed by consultant Atkins. It is a type of treatment that will convert sludge into compost similar to that available at the garden centre, by making the most of what nature has already provided in abundance - worms.

The creatures can convert sludge, either by eating the bacteria or solids within it, into a fertiliser bursting with the right types and concentration of nutrients that will not adversely affect the environment.

Using worms to treat sludge is far from new, explains Atkins development scientist Sophie Mormede. 'The use of worms to treat sewage sludge, or vermistabilisation, has been studied for decades, ' she says, adding however that this past research has usually concentrated on processing dried sludge cake. To adopt this traditional system, rural treatment works would have to install sludge drying facilities as well as a vermistabilisation plant, adding hefty costs to the overall solution.

Atkins has now removed the need for the dual approach by designing a vermistabilisation unit that can accept and treat liquid sewage sludge to make a usable fertilizer, says Mormede.

The unit, a 6m diameter circular tank filled with a base level of gravel for drainage and a soil bed above, is stocked with high quality worms capable of processing liquid sludge. The sludge is placed into the vermiculture unit in a controlled manner, via a rotating gantry.

The system is ideal for small or remote rural wastewater treatment thanks to its sustainability, adds Momede.

'There's no chemical treatment, very little power required and the end product can be transferred to the fields directly, ' she says.

The only extras the units may require is some insulation in very cold areas to protect the worms, she adds.

Once the sludge is mixed into the soil bed the worms go to work - fragmenting the sludge and improving its aeration due to their tunnelling action. The worms also boost nitrite, phosphorous, potassium and magnesium levels in the mix, perfect for plants. Importantly however, the worms also considerably reduce the sludge's unpleasant smell.

The tank need only be emptied twice a year, directly onto the fields together with some of the worms. 'The farmers like having the worms in the soil, ' explains Mormede. The remaining worms' fast gestation period means the population is soon built up again.

Atkins has a full scale trial unit in Yorkshire, and a plant already in operation in Northern Ireland.

South West Water and Scottish Water have also shown interest, as have food manufacturing factories keen to process food waste they produce in a more sustainable way.

'There has been a lot of scepti cism in the past about this, but it does work, ' Mormede says.

'And in the end, you get an odourless, useable product.'

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