Liquid and solids management impact on nutrient ratios for in-line wastewater prefermenters

Abstract

This research explored the effects of operational parameters on the prefermentation of primary wastewater sludge and the resultant wastewater characteristics changes at a full-scale in-line prefermenter, with due emphasis placed on the identification and resolving of practical problems. The settled sludge recycle flowrate was the main operational variable and the overall wastewater characterisation was based on the changes in the nutrients to carbonaceous material ratios across the prefermenter. The evaluation of prefermentation on wastewater nutrient ratio changes was performed under three different sludge operating conditions with regards to sludge elutriation rates, regulated at 0.7,2.2 and 3.7 kg sludge solids/m3 raw sewage. The corresponding sludge ages at these elutriation rates were calculated as 17.3, 2.5 and 7.5 days. At all three operating conditions the TKN/COD ratio increased (64, 46 and 20% respectively), with an average ratio of 0.057 mg N/mg COD in the raw sewage increasing to 0.082 mg N/mg COD in the settled sewage. These ratio changes limit the use and performance of certain biological nutrient removal process configurations. Linear relationships were identified for constituent removal levels across the prefermenter. Associations were based on solids removal (settleable solids vs. suspended solids), COD to solids removal (COD vs. suspended solids), nutrient to COD removal (TKN and TP vs. COD) and ammonia nitrogen change (ammonia vs. TKN) across the prefermenter. Solids (settleable solids and suspended solids) and COD removals against the prefermenter solids surface loading rates were also correlated. Although the operational conditions varied over the experimental stages, the scatter of data points still formed in most cases distinct linear trends that were quantified with linear equations. This study has also highlighted the importance to counterbalance the requirements of fermentation with the requirements of solids thickening and solids removal at in-line prefermenters. The sludge management function at prefermenters is crucial in cases where the downstream waste sludge handling capacity is limited, usually due to the absence of additional sludge thickeners. In the full¬scale prefermenter the minimum suspended solids removal (33%) was achieved at the lowest settled sludge solids content (4.25%), at a corresponding maximum volatile fatty acid process generation rate (5.7 mg VFA/l/h). These opposing results demonstrated the need to find a proper balance between the various operational fermentation and solids handling requirements. The solid and volatile fatty acid concentration profile down the prefermenter tank depth was determined at a high sludge blanket condition to demonstrate that the constituent mass inventory is about constant throughout the water and sludge layers respectively. The use of on-line sludge level detection instrumentation proved to be very beneficial to monitor sludge blanket height variations. The on-line data availability of the prefermenter sludge content simplified sludge age determinations. The increased solids loading on the prefermenter due to the internal sludge recycle places a limit on the level of thickening and solids removal that can be achieved in an in-line prefermenter. This aspect is not considered in detail in the available literature. Preliminary calculations at the case study site indicate that at the typical raw sewage suspended solids concentration (425 mg/l suspended solids), a solids loading of about 83 kg solids/m2 tank surface/d is created due to a sludge recycle rate of 0.05 m3 sludge/m3 raw sewage. This is related to an elutriation rate of 2.9 kg sludge solids/m3 raw sewage and a 5.6% total solids recycled sludge concentration, resulting in an APT feed concentration of 3158 mg/l solids. It is assumed that the recycled solids consist completely of resettleable solids. Unpredictable solids carry-over at peak flow conditions indicated however that loss of solids settleability and resuspension of solids do occur. Inhibitory substances can reduce the VF A generation performance of a prefermenter due to consumptive VF A counter-reactions. Escalating levels of sulphate in the raw sewage at the prefermenter was identified as a possible contributing factor for the reduction in the VF A content of the settled sewage over the experimental period. The surface scum collection system, transferring scum gravitationally with settled sewage from the fixed scum trough to the waste sludge sump, can dilute and lower the thickened waste sludge concentration unnecessarily. This problem was resolved by returning most of the scum transfer medium (settled sewage) with a submersible pump to the inlet of the prefermenter. The impact of wastewater prefermentation cannot be evaluated in isolation, based on the local prefermenter VF A production rate alone. The nutrient ratios and solids removal changes in the settled sewage from a prefermenter must be considered when evaluating the downstream biological nutrient removal process. The type of the prefermenter configuration employed is considered to be the determining factor to ensure sufficient operational flexibility is available at the primary treatment section of a water care works.