Management of Biosolids Recycling

Management of Biosolids Recycling

Scottish Water produces sewage sludge (Biosolids) as a natural by-product of the biological waste water treatment process. This sludge is then further treated to produce a biosolids product using various complex treatments such as thickening, dewatering, digestion, drying and lime pasteurisation.

Within Scottish Water, there are a number of sludge treatment centres (STC`s) employing various treatment techniques and manufacturing different biosolids products.

The schematic illustrates where these treatment centres are located (see fig 1).

In addition, a number of assets are operated by Private Finance Investment (PFI) concessionaires – nine schemes in total. PFI concessions treat 80% of sewage sludge (biosolids) generated in Scotland. The Daldowie facility treats 40% of the national total. A schematic illustrates the Scottish Water and PFI operated assets (see fig 2).

In 2017/18 the quantity of sludge generated was 120, 032 tonne dry solids(tds), the majority of which came from the PFI assets – 106,292 tds – and Scottish Water’s figure of 13740 tds.

The majority of the bio solids generated is recycled to agricultural land for beneficial re-use as illustrated in the pie chart  (see fig 3). Equally Scottish Water continues to support the restoration of land, in particular former open cast coal mines, with a both wastewater and water sludge cake. These sludges are blended together with other materials to produce a soil that is used to rejuvenate the land for use as public amenity areas, forestry and agriculture.

Biosolids have been recycled to agricultural land for many decades in the UK, Europe and the US, and such recycling is regarded as a safe and sustainable practice. This process is recognised as the Best Practicable Environmental Option (BPEO) in most circumstances, and this forms the cornerstone of Scottish Water’s sludge strategy.

Biosolids are rich in organic matter, and contain nitrogen and phosphate together with other important nutrients such as valuable trace elements that are useful in supporting crop growth and livestock nutrition. Biosolids are ideal soil conditioners owing to its humus forming and fertilising properties. In some cases significant savings on the cost of manufactured fertilisers can be realised. Regular applications of biosolid materials can improve water-holding capacity, drought resistance and structural stability, as well as the biological activity of soils. The greatest benefits are likely to be observed on soils where organic matter levels are low. Biosolids should be spread in rotation on all suitable land throughout the farm where agricultural benefit is likely, rather than on land which is conveniently situated in relation to steadings or roads. Care should be taken not to cause soil compaction, which will have a detrimental effect on crop growth and health, and may increase the risk of surface run-off.

Biosolids are not classified as waste. The application of biosolids to agricultural land is regulated by The Sludge (Use in Agriculture) (Amendment) Regulations 1990. The purpose of these regulations is to limit the build-up of heavy metals in soil and to restrict the planting, grazing and harvesting of certain crops following application. The regulations state that the sludge shall be used in such a way that account is taken of the nutrient needs of the plants and that the quality of the soil and of the surface and ground water is not impaired. Biosolids producers, like Scottish Water, are required to analyse field soils and sludge prior to application and to maintain records of applications of all sludge to agricultural land. Prior to storing biosolids, SEPA must be notified through the registration of a Paragraph 8 waste management licence exemption under WMLR. Clear guidance on the safe use of biosolids in agriculture and registration requirements is provided in the “Safe Sludge Matrix.”

More recently, The Biosolids Assurance Scheme (BAS) Standard, which is based on regulations and best practice, is audited by a third-party Certification Body to demonstrate that members of the scheme are conforming to its requirements. The resulting BAS Certified Biosolids should provide reassurance to food chain stakeholders and the public that these biosolids are safely and sustainably recycled to agricultural land. Scottish Water successfully gained certification to the BAS Scheme Standard in June 2017 for its mainstream sludge management operations.

Scottish Water offers soil sampling and recycling services which are provided through Scottish Water`s technical specialist staff and contractors, supplied to suit a farmer’s requirements.

Biosolids applications will be determined following soil sampling and analysis on individual fields, taking account of the soil pH, soil P and K status and the nutrient requirements of the following crops. Risk assessments will be completed to take account of the suitability of the area in terms of distance from watercourses, gradient of land and possible weather impacts, distances to domestic water supplies and dwellings and if any protected or conserved natural or historical features are nearby. If the land is within a NVZ other factors also need to be considered.

Changes in biosolids production technologies, in particular enhanced phosphate removal from waste water, and advanced chemical and thermal digestion processes may impact on the nutrient content of biosolids. There is a continuing need to ensure that the figures for the nutrient content of biosolids are up to date and representative of current practise at the farm gate. It is always advisable for the farmer/land owner to request an analysis report on a regular basis when receiving any type of biosolids product for on-farm use. Scottish Water maintains technical specification data sheets for each material that is beneficially recycled to agricultural land.

Scottish Water’s investment in waste water and sludge treatment has continued to increase, with a resulting improvement in the quality of the treated waste water and biosolids produced. Additionally, strict controls on industrial effluents discharged to the sewerage network have ensured that the quality of sludge meets all UK and European standards.

Scottish Water has been working on a longer term Bioresource Strategy which will consider the horizon to when all PFI concessions ends.

The strategy will have a clear focus on customer value, maximising renewable energy opportunities and strengthening relationships with local communities, farmers and key stakeholders.

Innovation will be a key theme for many aspects of this strategy across the whole bioresource journey from production, transport, treatment and recycling. The geography of Scotland is diverse and will require Scottish Water to look at both large scale technology solutions and small scale rural solutions that will consider the local communities that we serve.

The application of biosolids to agricultural land will continue to provide the best sustainable environmental option for Scottish Water through the implementation of the national strategy.

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