WASTEWATER TREATMENT, CARBON AND COST ON THE BOTTOM LINE.

Phil Black - PII Editor 11/06/2008

155 4 minutes read

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The Governments recently-issued “Future Water document, a DEFRA publication that maps Englands water management strategy to the year 2030, has as one of its core goals cutting the water sectors greenhouse gas emissions. Add to this a growing concern over the escalating costs of electricity and the focus puts low-energy wastewater technologies such as Brightwater FLIs HSAF Submerged Aeration Filtration firmly in the spotlight.

Another key driver is the Carbon Reduction Commitment Scheme introduced in the 2007 Energy White Paper, which will apply mandatory emissions trading to cut carbon emissions from large commercial and public sector organizations. Due to be introduced over three years from 2010, the scheme will definitely encompass the water sector, which is a substantial energy user, and will introduce carbon allowances that will be sold at a fixed price of £12 per tonne of CO2 emitted. Participants will have to purchase allowances corresponding to their emissions, a powerful incentive for large users to drive down their energy use. As wastewater treatment is a major element of the sector’s power consumption, the highly-efficient HSAF® system’s minimal air and power requirements will make it a technology of considerable interest.

Brightwater FLI’s HSAF® process is designed to enable BOD and/or ammonia reduction, depending on its configuration within the overall wastewater plant treatment processes. In most applications, treatment streams are fed through the base of the packaged unit to an aerated biological treatment zone containing the company’s advanced BMax® media.

Here, wastewater flows upwards through the buoyant recycled plastic media, whose contoured surfaces host a high concentration of active bacteria that reduce BOD or ammonia levels as required. The air supply that feeds the biological process is distributed evenly throughout the tank via an air grid at its base – this efficient oxygen transfer system means less air is used and less power is required than in standard systems. The efficiency of the oxygen transfer enables transfer rates of 8 to 10% per metre of tank depth compared to 4 to 5%/m in a standard structured media plant.

The cleaning cycle consists of an air scour and one or more simple desludge stages. The air scour phase passes additional air through the media, creating turbulence and gently fluidising the media for around ten minutes at intervals of typically between two and seven days, thus releasing the excess solids and biomass.

The reactor then undergoes a settlement period with the air turned off, so that the media bed can repack and the solids collect at the base of the reactor. The excess solids are then removed from the base before the reactor is put back online. This procedure maintains media bed hydraulic capacity and enables retention of the biomass, which is needed to ensure treatment efficiency is maintained.

The Brightwater FLI HSAF® tank’s compact construction and low energy input ensure that system has very low carbon and physical footprints. Cost comparisons with other equivalent technologies reveal that many rival processes have much higher power requirements. For instance, a conventional structured media system has a power requirement of 3.7kW at 300mbar of air pressure, whereas the power requirement for the HSAF® at the same pressure is just 1.5kW, and the cost per annum for process air at 11p/kwh is £1445 ($2855), substantially less than the £3565 ($7043) energy bill for the structured media system. This would result in a £42,400 ($83,765) saving over the typical 20 year life of such an asset.

The figures for an alternative random media SAF are 3.5kW and £3372 ($6661) per year, giving a £38,540 ($76,134) saving over 20 years with a Brightwater system, with similar results for mineral media SAF systems.

In addition, power to provide scour air for backwashing in the HSAF® costs just £5.68 ($11.22) a year as the buoyant media can be fluidised to release excess solids with minimal additional air.

In terms of performance, the HSAF system also outshines rivals despite its modest power use. It reduces BOD and ammonia levels in wastewaters using the versatile static bed biological process and BMax® floating media, whose high specific surface area (700m2/m3), is more than three times that of structured media systems, which generally provide a surface area for treatment of around 200m2/m3.

The exceptional surface area enables the BMax® media to retain a high bacterial population, which in turn facilitates higher BOD loadings of 2.3kg/m3 to achieve a 20mg/l BOD standard, compared to 1kg/m3 for a structured media system. Focusing on the system’s other removal option, the ammonia loading rate for 5mg/l is 0.3kg/m3 compared to a far inferior 0.08kg/m3 in a structured media system.

Despite the HSAF system’s low power requirement and tiny footprint, this rich bacterial environment means the system is able to produce high-quality effluent containing just 1mg/l of ammonia in tertiary mode. Performance data from a range of plants in operation shows that suspended solids of less than 15mg/l at the 95 percentile can also be readily achieved at flow rates of up to 6m/hr.

The Brightwater HSAF technology is well proven, with over 40 units installed in both municipal systems and non-municipal applications such as landfills. The filter is available in a range of package plant sizes from 1m2 to 12m2, all with a reactor depth of 2.5m, to treat flows from 3litres/sec to 20 litres/sec.

Fore more information contact Brightwater FLI Ltd, Letchworth, Herts.
Tel: +44 (0)1462 485005 Fax: +44 (0)1462 485003
Email: enquiries@brightwaterfli.com Web: www.brightwaterfli.com