Bacterial behaviour in an aerobic treatment lagoon at cooler temperatures: investigation into a decline in treatment efficiency during winter at Amcor Cartonboard Petrie Mill

A strong inverse correlation between temperature and organic load is present in the Petrie Mill main aerobic treatment lagoon which treats paper machine effluent. Lower temperatures experienced in winter months correspond to an increase in COD results at the pond outlet. This research and investigation was aimed at improving effluent treatment during cooler temperatures, by determining the extent of the impact of temperature in the treatment of effluent, as well as the influence of other factors such as oxygen or nutrient demand. Investigations and laboratory trials concluded that temperature did not directly impact the ability of the bacteria to consume the organic load. Ingestion of organics into the lagoon bacteria population occurred at the same rate across the temperature range tested. However, the dynamics of load reduction differed in a warmer versus a cooler environment. During lower temperatures, the catabolic requirements of bacteria are reduced, and the synthesis component of bacterial metabolism becomes an essential aspect of achieving the desired load reduction. The organic load was able to be effectively reduced at low temperatures, provided conditions were adequate for bacterial synthesis to occur. Temperature did not appear to affect bacterial synthesis. The impact extent of lower temperature appeared to be a reduced ability of excreted enzymes in biodegrading complex organic material. The major factors influencing organic load removal at lower temperatures were found to be those inhibiting bacterial synthesis, which were dissolved oxygen and nutrient availability. In the absence of adequate oxygen or nutrient supply, synthesis occurred at a lower rate or did not occur at all, leading to the poor treatment of effluent load at the low temperature. The investigation also included that the lagoon sludge bed had an impact on treatment during winter. Nutrients are absorbed by the sludge bed in cooler temperatures, contributing to possible nutrient deficiencies in winter. This would lead to reduced treatment efficiencies. Load variation also impacted on treatment performance, again due to higher organic loads without a corresponding increase in nutrient or oxygen availability for satisfactory bacterial synthesis. Toxicity had an inhibitory effect on bacteria functions. A number of immediate changes and have been made as a result of this investigation. Changes were based primarily on improving dissolved oxygen and nutrient availability, including redistribution of aerators and increasing nutrient dose rates. As a result the COD load at the lagoon outlet was reduced by approximately 20% and the system appears to be in a stable and robust condition entering the current winter period. It is anticipated that the implementation of recommendations for future improvement will further improve performance and robustness of the treatment system.