The grass is greener...

Yorkshire Water treats around 350M m3 waste water per year, resulting in the production of 150,000-plus tonnes of dry solids. By 2010 this production will have increased by at least 15% due to improvements made at many of the company's 600 wastewater treatment works to meet the Freshwater Fish Directive requirements. Tom explains.

Yorkshire Water is always keen to ensure that it works with the environment in mind and, over recent years, has been developing the use of sludge phyto conditioning to provide enhanced sludge, or biosolids.

As a result, we have been investigating a number of options for the future, keeping ahead of the legislation and building a robust plan that should also lead not only to a safer environment but also to reduced costs.

When most other water companies were looking at pre-pasteurisation as a way of creating an enhanced treated sludge, we began to develop sludge phyto conditioning (SPC).

The aim was:
  • To plan for future tightening of legislation
  • To secure new recycling opportunities only available with enhanced treated sludge
  • Reduce overall costs
  • Have a positive impact on the environment, meeting new legislation such as the Nitrate Vulnerable Zones Directive, Sludge Directive and Sludge (Use in Agriculture) Regulations.
Most of the Yorkshire Water sites with sludge recycling treatment facilities use the digestion/conditioning process, representing approximately 70% of sludge to land. The process is very good at achieving conventional treatment, but sludge phyto conditioning is a simple, flexible treatment concept that takes conditioned sludge to the enhanced level, using only the assets that are already in operation: digesters, dewaterers, conditioning plant and land.

Reduced costs
Over a two-year period we worked with Atkins Water and Mel White, carrying out a structured programme of tests to demonstrate that growing grass on conditioned sludge could promote a number of beneficial changes in sludge composition. The trials, which were undertaken with our consultants and also in-house, ensured that the Environment Agency (EA) could validate the treatment process, of vital importance if we were to take this treatment method forward to large scale operation.

The method we use is simple. We take raw sludge and process it through the usual method for sludge cake, but it is at this point where the company works in partnership with other organisations to get the best possible produce.

We work with partner Steve Butterfield, whose company collects green waste from most of Bradford's refuse collection areas, along with a few in the Huddersfield and Leeds areas.

The waste, which is made up of tree and grass clippings, garden waste and logs, is then shredded and mixed with the sludge cake.

Once conditioned, the sludge is laid out and seeded with an annual rye grass, Lolium multiflorum.

During the trial a number of plant species were used and L multiflorum was eventually chosen as the most appropriate because it is very quick growing and good at keeping weeds at bay. Its root mass was also just right for the process.

The ryegrass is grown on the stacked cake for one season, during which time a number of things take place: root penetration of the grass into the cake causes enzymic degradation and aerobic conversion; and the pathogens within the cake are starved through competition and removed through predation leaving an enhanced sludge which is desiccated through the transpiration of the grass growth.

The process achieves a product that has reduced pathogen levels to such an extent that E coli is no longer detectable within four months of grass growth. It also achieves a 40% reduction in sludge volume, considerably cutting the associated transport emissions and reducing costs.

Easier disposal
One of the main benefits of the treatment system is that sludge phyto conditioning can act as a back-up solution where conditioning fails its HACCP or final concentration standards.

The sludge can be put through the process, ensuring a higher quality final product of the required standards and because the process is batch in nature, it is easily auditable. As the final product is enhanced, it is much easier to dispose of, with a number of contracts already in place for use within the agricultural and horticultural environment.

One such contract belongs to Elizabeth J Design landscape gardens, which has designed gardens around the use of enhanced sludge with recycling at the forefront of design planning. Under Royal Horticultural Society conditions, the designers have won two silver guild awards, one each at the last two Harrogate Flower Show events.

Here the biosolids provide an excellent top soil alternative or soil conditioner. They are also used as an alternative to peat in garden products.

Currently around 35,000tds of biosolids is recycled onto 6,500 hectares of agricultural land annually and it is recognised in both the UK and the EU to be the best environmental option in most cases. Legislation ensures only quality products are recycled in this way, giving confidence to consumer groups including the British Retail Consortium and the Food Standards Agency.

The trials have now been completed and we use sludge phyto conditioning at a
number of our wastewater sites across the Yorkshire region. And with sludge production in the increase due to higher levels of treatment, this simple but highly effective treatment method is definitely the way forward for Yorkshire Water.
Tom Taylor is senior project engineer at Yorkshire Water.
T: 01274 804040.

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