Getting to the gritty part

As water companies prepare to increase the opportunities for beneficial recycling of sewer grits, best practice guidance is being prepared for the water industry write Kathy Lewin, Tony Dee and James Peacock of WRc

More than 40,000t/yr sewer grit is generated from wastewater treatment works (WwTW) in the UK and a high proportion is landfilled. Spring 2011 brought both a further increase in landfill tax and implementation of the revised Waste Framework Directive in the UK. These are driving the need to consider this gravel-rich material as a valuable resource.

Some water companies are conducting grit recycling operations but sewer grit is not exploited in the UK to the same extent as it is in Europe.

The Environmental Permitting Regulations (Statutory Instrument 2010, No. 675) appear to pose a barrier to their recycling. From October 2011 sewer grit can no longer be used under an exemption from permitting for land reclamation or construction projects outside the curtilage of the WwTW or sewer network.

However, under the Waste Regulations (Statutory Instrument 2011, No. 988), which implement the revised Waste Framework Directive (2008) and came into force on 29 March 2011, the water industry must take all reasonable measures to follow the hierarchy of waste management options (Figure 1). This prioritises waste prevention and recycling ahead of disposal.

With higher grit disposal costs and a legislative requirement to 'reduce, reuse and recycle', there is a strong incentive for the water industry to take advantage of sewer grit recycling opportunities.

Grit production
Sewer grit collected by traditional removal plant at WwTWs consists mainly of mineral matter such as sand, gravel, glass and plastics. Grit production is highly variable (0.5 to 2.0t/yr per 1000 population) depending on the catchment area (higher grit production in coastal or sandy areas), sewer type (more from combined sewers through road run-off than separate sewers), and weather conditions (grit deposited in the sewer during dry weather is washed through during storm events). Grit quality has wide variations in moisture content (7.4% to 75%) and organic content (30-65% total organic carbon). Owing to the high proportion of sand and gravel-sized inorganic material, sewer grit has been used in several recycling applications. Untreated grit has been used as top cover / infill for pipe trenches within the curtilage of wastewater treatment works during construction projects.

Certain soils, particularly heavy clay soils, can benefit from the addition of grit materials to aid in drainage, to help break-up those soils and to improve compaction properties. It may be used in horticulture and agriculture.

Following a successful trial by Dŵr Cymru Welsh Water with a local soil supplier, Neals Soils Supplies, and the Environment Agency, grit from the Cardiff and Gwent catchments is used as a base material for soil manufacture. Grit has also been successfully trialled as an input material for the cement industry as an alternative source of silica rich material.

Grit washing
Washing to remove residual organic material improves prospects for disposal of sewer grits. After washing, grit has potential for disposal to inert waste landfill with lower gate fees and tax than non-hazardous waste landfill. Manufacturers claim that modern washing plants typically give approximately 40% weight reduction, up to 60% volume reduction; loss on ignition of less than 3% and an increase in dry solids (DS) to approximately 90%. Opportunities exist for recycling of washed grit as secondary aggregates, particularly after grading, and if compliance with WRAP's protocol for inert aggregates is achieved.

The washed material needs to be equivalent in physical ( for example, grading) and chemical (for example, contamination) characteristics as the virgin aggregates it seeks to replace, so it needs to be compared with specifications for virgin aggregates.

The sustainable recycling hub at Coleshill WwTW, which treats sewer grit from outlying works and imported road sweepings, in addition to the site's own production, generates a washed grit suitable for recycling as aggregate.

Such grit-washing systems are adaptable as they can be retrofitted to existing plants, installed as new plants or scaled to treat grit regionally from many sites.

Grit composting
Under aerobic conditions and in admixture with other waste, the organics which are present in sewer grit are biologically decomposed. The production of heat further sanitises the input material.

In addition to sanitisation, the composted product is stabilised and becomes an important source of nutrients and humic substances that can be applied to land. Mixing the organic input with green or woody wastes is often carried out to optimise the process, which takes in the region of 12 days to complete. Adequate segregation of non-compostable items is required before and/or after composting.

Under the revised BSI PAS100 protocol (2), composting of sewage sludge and its derivatives, such as grit, is no longer allowed in compost products. However this does not prevent its application as a soil improver on agricultural land under an exemption through Environmental Permitting.

Anglian Water has a facility at Great Billing WwTW in Northampton for composting green waste with washed grit.

Composting Facilities Services operates a recently-installed plant for co-composting of grit and screenings at Kingsnorth, near Rochester in Kent.

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