Linking up nutrient removal, liquor treatment and nutrient recovery
Narroway, Y., Barker, R., Vale, P. and Chadha, M., Severn Trent Water, UK
(free)
Severn Trent Water (STW) will have over one hundred sites with Total Phosphorus (Total P) consents by the end of AMP5. STW’s agreed approach is to adopt Enhanced Biological Phosphorus Removal (EBPR) wherever possible and both our biggest sites, Minworth and Stoke Bardolph, are being converted to an EBPR configuration. To minimise costs and impact to EBPR, the high strength digested sludge liquors at these sites are to be treated separately in specialist processes. This allows a holistic approach for potential P recovery as struvite. Pilot trials have been carried out on both ammonia removal and P recovery from the liquors, the results of which have led to the installation of the first UK ANAMMOX® plant at Minworth and a planned installation at Stoke Bardolph, combined with a P recovery process where struvite is harvested.
Keywords: Anammox, Anaerobic Digestion, Phosphate, Ammonia, Recovery
Introduction
Severn Trent Water (STW) is a UK water company providing water & sewerage services to around 8.5 million people in the Midlands and mid Wales. By the end of 2015, STW will have 50% of its wastewater undergoing some form of Enhanced Biological Phosphorus Removal (EBPR). In this way Phosphorus is removed biologically rather than by chemical precipitation. This strategy has allowed a more sustainable approach to Phosphorus removal with a reduced amount of chemical usage.
In STW, virtually all of the sludge generated in the wastewater treatment process undergoes mesophilic anaerobic digestion (MAD). High strength liquors from MAD are generally returned to the head of the main treatment process and can have a significant impact on treatment. The ammonia load from MAD liquors typically contribute 25% to the total ammonia load into the main treatment process. This load requires considerable aeration capacity and can be expensive to treat. There is also an impact to EBPR plants in particular as the returned activated sludge will contain increased levels of nitrate which can be detrimental to the EBPR process (Janssen et al, 2002).
In Severn Trent, where there has been insufficient capacity to treat liquors in the main treatment process, there have been liquor treatment plants (LTP) constructed to treat this high strength waste. These have been based on conventional activated sludge processes configured for complete ammonia removal and partial denitrification. The plants have fulfilled a need but have been energy intensive processes, have requiring chemical addition to replenish alkalinity. The installations have also continued to return high levels of nitrate to the EBPR process.
In addition to a high ammonia load it has been observed that MAD liquors can contain elevated levels of phosphorus. This is particularly true when EBPR is employed on site (Batista et al, 2006.) as polyphosphate contained in phosphate accumulating organisms (PAOs) can be released as orthophosphate when EBPR sludge is digested. It is found that the elevated P levels in the liquor can add 30% to the P loading entering the main treatment process. This has a significant impact on the EBPR process and there is typically not enough carbon available, which drives EBPR, to remove this additional load.
Perhaps of greater concern is the potential of the Phosphorus rich liquors to form scales of the mineral struvite (MgNH4PO4·6H2O – magnesium ammonium phosphate or MAP). This scale is well known for causing severe operational problems by blocking pipes and pumps which is very expensive to resolve. However, Phosphorus is also an essential nutrient for life and it is used widely in fertilisers, detergents, foodstuffs, medicines, fire retardants and more. It is therefore advantageous to be able to harvest this key nutrient from sewage sludge in a sustainable way for future reuse. By widely employing EBPR, STW has reduced operational expenditure on coagulating chemicals and may also be able to recover the phosphorus from the resulting digester liquors which are rich in phosphorus.
This paper looks at two sites within STW which have been configured for EBPR in AMP5; Minworth Sewage Treatment Works serving Birmingham and Stoke Bardolph Sewage Treatment Works serving Notttingham. The liquor treatment plants for these sites have been chosen to best tie in with the main EBPR process at reduced OPEX and CO2 emissions.
