Proceedings

Abstract

Within the UK and Europe the needs for removal of the nutrients nitrogen and phosphorus, is driven by the requirements of the EC Urban Wastewater Treatment Directive. The majority of UK treatment plants have an ammonia consent imposed by the Environment Agency which requires them to achieve nitrification. It is standard design practice in such circumstances to install a denitrification zone to reduce the effluent nitrate concentration which is known to aid the settleability of the sludge. Both biological and chemical phosphorus removal is practiced in the UK and whereas not common, the number of plants that must achieve biological phosphorus removal is quickly increasing. This presents a challenge in the UK where wet winters generate a weak sewage that cannot provide adequate carbon to drive both denitrification and biological phosphorus removal. Then, a successful degradation process requires an additional carbon supply to be provided for the denitrification stage and this is the circumstance where methanol or another external carbon source is bought and added to the process for this purpose. Sonication of the excess sludge with ultrasound breaks down the biomass and releases the cell content -ideal carbon carriers- which are then available as an internal source of carbon in the denitrification stage. Biological nitrogen degradation in the wastewater treatment plant can therefore be maintained or even intensified. If part of the sonicated sludge is returned to the aerobic phase, the quantity of sludge to be disposed of is automatically reduced and bulking and foaming problems are complete eliminated. This is the application where Ultrawaves aims to optimise the biological reactor through the transformation of a waste into a resource and through other collateral benefits as well. This paper pretends to be descriptive and tries to show a systemic outlook regarding this efficient and innovative solution.

Keywords:  Ultrasound, denitrification, biomass, carbon carriers, excess sludge, nitrogen degradation, bulking and foaming

Introduction Continuous research and development has created Ultrawaves ultrasound technology new and revolutionary applications in aerobic processes. There has been a big effort to understand the best way to optimise a biological reactor with ultrasound which has now been reached through different applications. It is now possible the complete elimination of associated problems with bulking and foaming through a selective sonication of a very tiny amount of Returned Activated Sludge (RAS). In addition it is also possible to enhance a dramatic excess sludge reduction when a partial thickened waste activated sludge (TWAS) flow is disintegrated and returned back to the biological reactor whenever a cell lysis and cryptic growth process is induced. Sonication causes cell lysis with the consequent solubilisation of cellular constituents which become substrate available for further biodegradation which in turn results in an overall reduction of the excess sludge production (Hamer and Mason 1987; Canales et al., 1994). The most recent and interesting application has been carried out in the biological nitrogen removal due to sonicated TWAS performs a very useful and autochthonous readily biodegradable carbon source to denitrify. When TWAS is sonicated and recycled back to the anoxic zone of the biological reactor is used then as carbon source to support denitrification and the facility transforms a waste into a resource with a subsequent saving in carbon source purchase and sludge disposal cost.

It has been demonstrated that low frequency ultrasound waves generate the cavitation necessary to produce mechanical shear forces associated with sludge disintegration. Combined with high intensity ultrasound, the cell aggregates as well as single cells are destroyed and enzymatic and intracellular material is released into the medium resulting in a higher degree of substrate bio-availability for the remaining living microorganisms. In effect, the enzymatic biological hydrolysis, which is the initial and rate limiting of the biological food chain, is substituted and catalysed by this mechanical disintegration of the sludge (Tiehm et al., 2001).

Increasing its bioavailability with Ultrawaves technology, disintegrated sludge can be used then as an internal electron donor (energy source) to fuel denitrification stage. Indeed Biological Nutrient Removal (BNR), which is considered as the most sustainable and efficient process to remove nutrients from wastewater needs a sufficient carbon concentration (Abu-ghararah and Randal 1991) to be successful which causes a constant carbon sources demand. Hence the addition of chemicals such as methanol or acetate to enhance BNR process is sometimes necessary but the associated costs are significant and the need to have chemical storage facilities on the site decreases its attractiveness. However through excess sludge disintegration the release of suitable electron donors can be promoted and this by way sludges are re-used decreasing the overall sludge production in a facility while the BNR efficiency is assured.