Abstract
From a theoretical investigation of the continuity of wastewater organic (COD) and N compounds along the links connecting the primary settling tank (PST), fully aerobic or N removal activated sludge (AS) treating raw and settled wastewater and aerobic digestion unit operations, it was found that the PS characteristics, viz. the biodegradable and unbiodegradable soluble and particulate COD and N concentrations, need to be calculated from mass balances around the PST so that the organic and N concentrations conform to continuity principles. Also, it can be accepted that the influent wastewater (fixed) inorganic suspended solids (ISS) concentration is conserved through the primary settling tank, activated sludge, aerobic digestion systems. However, the measured ISS flux at different stages through a series of wastewater treatment plant (WWTP) unit operations is not equal to the influent ISS flux because the OHO biomass contributes to the ISS flux by differing amounts depending on the active fraction of the VSS solids at that stage. The steady state activated sludge and aerobic digestion models, both modified to include the inorganic suspended solids (ISS) and the latter to include aerobic digestion of primary sludge, yielded virtually identical results as Activated Sludge Model No 1 (ASM1), also modified to include the ISS. This research shows that the mass balance-based steady state activated sludge and aerobic digestion models, modified to include the ISS compound, can be coupled to produce a plant-wide WWTP model for aerobic stabilisation of sludge that can be used for design and operation and checking of simulation model results. Water SA Vol.32 (3) 2006: pp.297-306
Highlights
From a theoretical investigation of the continuity of wastewater organic (COD) and N compounds along the links connecting the primary settling tank (PST), fully aerobic or N removal activated sludge (AS) treating raw and settled wastewater and aerobic digestion unit operations, it was found that the primary sludge (PS) characteristics, viz. the biodegradable and unbiodegradable soluble and particulate COD and N concentrations, need to be calculated from mass balances around the PST so that the organic and N concentrations conform to continuity principles
It follows from the above that: - the PS characteristics need to be calculated from mass balances around the PST so that the organic (COD) and N concentrations and components, viz. biodegradable and unbiodegradable, soluble and particulate, conform to continuity principles, - the influent unbiodegradable particulate organics determined from response of the activated sludge system are unbiodegradable under anaerobic digestion conditions, - the unbiodegradable particulate organics that are generated in the activated sludge reactor, i.e. endogenous residue, are unbiodegradable under anaerobic digester conditions and - the residual biodegradable particulate organics that can be anaerobically digested can be calculated from the active fraction of the waste activated sludge (WAS) using the widely accepted stoichiometric and kinetic constants in AS models such as Activated Sludge Model No 1 (ASM1) (Henze et al, 1987)
From the experimental data in several literature sources, it is reasonable to accept that the influent wastewater inorganic suspended solids (ISS) concentration is conserved through activated sludge and aerobic digestion systems
Summary
111.8 nitrifying aerobic digestion of waste activated sludge there always should be sufficient ammonia for growth because the ammonia released in OHO death always is greater than that taken up for OHO growth on the slowly biodegradable (SB) COD released in OHO death. In application of the model, nitrate utilisation for OHO growth can occur This can occur with model application to aerobic digestion of primary sludge even when sufficient ammonia is dosed for net OHO biomass production from the PS particulate biodegradable organics. This apparent ammonia deficiency in the model arises because the rate of nitrification is faster than the rate of OHO growth, which is limited by the supply rate of SBCOD from the OHO death process.
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