K5/1822 - Mass balances modelling over wastewater treatment plants III 
Funded by: Water Research Commission
April 2008 – March 2010

Partner organisations: University of Cape Town; eThekwini Water and Sanitation 

Project description
In 2008, The Pollution Research Group joined the University of Cape Town’s Water Research Group in a continuation research study to develop mass balance models for wastewater treatment plants (WWTP). Since 2002, UCT has received funding from the Water Research Commission (K5/1338, K5/1620) to develop mass balance models that account for the complex set of operations and processes that occur in wastewater treatment systems. Through these studies, the UCT developed a comprehensive range of steady state and dynamic simulation models, including steady state anaerobic digestion (COD, C, N, O, H) model; and a mass balances based steady state model for the entire WWTP comprising primary settling, ND activated sludge and aerobic or anaerobic digestion of primary and waste activated sludge. such models for WWTP that account for the complex set of operations and processes that occur in wastewater treatment systems. 

In 2008, UCT received a third round of funding from the WRC to continue their investigation into mass balances models (Mass Balances III; K5/1822). Pollution Research Group collaborated on this project, providing the expertise in bioprocess modelling. The Mass Balances III followed up on research questions not considered in WRC/1620 such as the behaviour of the phosphorus accumulating organisms (PAO) from biological excess P removal (BEPR) activated sludge systems under anaerobic digestion conditions; and whether anaerobic digestion of WAS together with primary sludge (PS) has a beneficial effect on the hydrolysis rate of WAS compared with anaerobically digesting the WAS by itself.

The objectives of Mass Balances III were to:
  • Determine the kinetics of P release from biological excess P removal (BEPR) waste activated sludge (WAS) under anaerobic (AD) conditions.
  • Establish to what extent mineral precipitation takes place in BEPR WAS ADs
  • Compare the P release in AD with that observed in aerobic digestion in WRC 1620.
  • Confirm mineral precipitation in aerobic digestion with parallel aerobic digestion tests
  • Confirm un-biodegradable fraction of primary sludge with two sources: mass balance round primary settling (PST) and primary sludge (PS) AD. 
  • Confirm un-biodegradable fraction of nitrification-denitrification (ND) system WAS from the modified Ludzack-Ettinger (MLE) system performance data and AD of MLE system WAS for both raw and settled wastewater.
  • Measure the hydrolysis rate of WAS co-digested with PS
  • Develop and extend existing activated sludge, aerobic and anaerobic digestion models towards assembling them into plant-wide WWTP models. 
The Mass Balances III study comprised extensive experimental research into the characteristics of sludge and wastewater that is found in WWTPs, as well as continued model development for steady state and dynamic simulation activated sludge and aerobic and anaerobic digestion models using Aquasim and WEST computer platforms. In addition to the experimental work and model development, three sub projects were carried out to answer municipal WWTP requests that arose in Mass Balances II. The first examined the biodegradability of toilet paper and its impact on sludge production at WWTPs. The second assessed the impact of changed levels of organics in wastewater on the biological N and P removal performance of the activated sludge system. The third sub study focused on WWTP sludge production minimisation practices and technologies.