Abstract
The water industry is facing more stringent Environmental Quality Standards (EQS) as the Water Framework Directive (WFD) dictates good chemical and ecological status in all water bodies. In the meantime, Ofwat’s rigorous pricing approach requires high efficiency from the water companies, with a particular focus upon whole life cost (WLC) and reducing both embodied and operational carbon emissions. Water companies are incentivised to outperform and deliver the low Totex solution that is also robust and reliable.
Most of the consent requirements, such as TSS, COD, BOD, Total Phosphorus (TP) and Total Iron (Fe), all rely on a very low effluent suspended solids concentration. Therefore, efficient solids removal technology is becoming increasingly critical in AMP6 for many sites with stringent consent limits.
Bluewater Bio’s FilterClear has been selected on Anglian Water’s Framework for Tertiary Solids Removal after comprehensive trials. Three FilterClear plants have since been installed and commissioned, treating FFT from 5 L/s to over 50 L/s.
Because of its excellent solids removal capability and low Totex, it is anticipated that FilterClear will be a suitable option for TP removal in AMP 6. The package plant design and offsite manufacturing enables quick installation and mobility, also makes FilterClear perfectly suited for troubleshooting and emergency response.
Keywords: Filtration rate, media stratification, Totex, Runtime, Backwash, Water Framework Directive, Carbon emissions, Removal of TSS, BOD and TP
Introduction and Regulatory Background
The current water policy (such as Urban Waste Water Treatment Directive and Drinking Water Directive, etc.) was considered to be fragmented, and only tackling individual issues (Environment Directorate-General, European Commission, 2015). The Water Framework Directive (WFD) was adopted and came into force in December 2000, aiming to achieve “good ecological and chemical status” for all waters, inland surface waters (rivers and lakes), transitional waters (estuaries), coastal waters and groundwaters.
One advantage of the WFD approach is that it will rationalise and streamline the water legislation by replacing seven directives, including; the freshwater fish, shellfish waters, groundwaters and dangerous substances directives (Environment Directorate-General, European Commission, 2015).
The Water Framework Directive was transposed into UK law in 2003. The Agencies are now reviewing the River Basin Management Plans for the 2nd cycle (2015 – 2021), and developing new or revised environmental standards based on the latest understanding of scientific and technical evidence (Defra, 2014).
Among many revised standards, the UK Technical Advisory Group (UKTAG) found the standards for phosphorus in rivers set in 2009 were not sufficiently stringent. “In 75% of rivers with clear ecological impacts of nutrient enrichment, the existing standards produce phosphorus classifications of good or even high status.” (Defra, 2014) The proposed new phosphorus standards are accordingly more stringent than the existing standards.
Regarding discharges from sewage treatment works (STWs), most of the consent requirements, such as TSS, COD, BOD, Total Phosphorus (TP) and Total Iron (Fe), rely on a very low effluent suspended solids concentration. Therefore, efficient solids removal technology is becoming increasingly critical in AMP6 for many sites with stringent consent limits.
Over the past 30 years, a wide variety of treatment technologies have been developed and applied for the removal of the residual contaminants in secondary treated effluent. Removal of organic and inorganic colloidal and suspended solids is typically achieved by filtration. The filters used for wastewater can be classified into three categories (Metcalf & Eddy, 2004): depth filtration, surface filtration and membrane filtration.
To achieve a BOD concentration less than 10 mg/l, the particulate BOD has to be less than about 7 mg/l, allowing 3 mg/l soluble BOD. This means the effluent TSS has to be below 10 mg/l, which will require a suitable tertiary solids removal process. In general, such stringent BOD and TSS consents will entail MBR or deep bed sand filters in many water companies’ process selection matrices.
MBR, although it will guarantee good solids removal, will render most of the existing assets redundant, and is considered to be a high Capex and Opex option. Deep bed sand filters (rapid gravity filters) are expensive to build, occupy a relatively large footprint and cannot always guarantee the required effluent quality. The development of continuous backwashing filters (CBFs) in the 1990s offered some advantages over traditional rapid gravity filters in terms of a smaller footprint and reduced construction costs, particularly for smaller sites, but CBFs have high operating costs and will seldom be the lowest WLC solution.
