Two options were studied in order to meet new BOD effluent limits at this small WWTP: build additional lagoons or add an MBBR in a LagoonGuard® configuration.
An AnoxKaldnes™ LagoonGuard® MBBR technology was selected and constructed.
Technology Design Data
The existing lagoon treatment plant consists of four lagoons of respectively 41,400 m3, 29,800 m3, 28,460 m3 and 16,700 m3. Total Actual Oxygen Requirement (AOR) aeration requirement is of 1,290 kg O2/d. Expansion would require an extra 115,140 m3 lagoons volume and an additional AOR of 778 kg O2/d.
Average daily wastewater flow is 16,700 m3/d. Influent BOD to the MBBR is 69 mg/L, ammonia is 16 mg/L. Treated required BOD is 26 mg/L. There is no requirement for ammonia treatment at this stage but another MBBR tank will be installed later to meet the requirement.
Installed MBBR was based on LagoonGuard® treatment system. Effluent of the first lagoon is fed into one MBBR tank. Effluent from the MBBR will feed the second lagoon. The MBBR reactor volume is 1,256 m3. The MBBR media chosen for this application is AnoxKaldnes™ type K1 media for a total volume of 626 m3.
For the evaluation of the LagoonGuard® MBBR, the items taken into account were the concrete poured for the reactor, the structural steel used for the reinforced concrete, the steel used for all ancillaries (sluice gates, blowers, piping, etc.), the PVC used for the K1 media and the electricity costs over a 20 year evaluation.
As for the additional lagoons, the excavation costs and the weight of ancillaries (sluice gates, piping, blowers, etc.) were evaluated, as well as the electricity costs for a 20 year evaluation.
Because of the huge volumes of earth being manipulated and the specific geotechnical considerations of the ground (clay, earth, rock, etc.) the evaluation of the carbon footprint of the excavation will be of paramount importance when lagoons are being compared. Two extreme situations were considered: the best case scenario (only earth, no long distances of transportation, etc.) would require 3 kg CO2/m3 of earth moved whereas the worst case scenario (rock, longer distances of transportation, etc.) would require 18 kg CO2/m3 of earth moved (UK reference).
Carbon Footprint per Cubic Meter of Water Treated*
Carbon Footprint over Life Expectancy of the System*
Much of the sensitivity of the analysis in this case study rests with the intensity factor that would be chosen for the excavation during the construction phase. When the carbon footprint of the excavation is taken to be 3 kg CO2/m3, the LagoonGuard® MBBR and the additional lagoon have very similar carbon footprints. However, when the excavation becomes more complex, its impact becomes greater in the overall construction footprint. At 18 kg CO2/m3 (UK reference), the construction of the lagoon is close to 4 times more carbon footprint intensive than a LagoonGuard® MBBR, over a 20 year period.
The physical footprint of both solutions must also be weighted: the MBBR would require about 300 m2 of area whereas the lagoon would need around 40,000 m2 of area, making the MBBR the preferred choice when looking at retrofitting lagoons in constrained locations.