Evaluation of a new model for the reduction of excess sludge production by ozonation of return activated sludge: what solids COD fraction is affected?

Sludge reduction by ozone: Insights and modeling of the dose-response effects

Applying ozone to the return flow in an activated sludge (AS) process is a way for reducing the residual solids production. To be able to extend the activated sludge models to the ozone-AS process, adequate prediction of the tri-atoms effects on the particulate COD fractions is needed. In this study, the biomass inactivation, COD mineralization, and solids dissolution were quantified in batch tests and dose-response models were developed as a function of the reacted ozone doses (ROD). Three kinds of model-sludge were used. S1 was a lab-cultivated synthetic sludge with two components (heterotrophs X_H and X_P). S2 was a digestate of S1 almost made by the endogenous residues, X_P. S3 was from a municipal activated sludge plant. The specific ozone uptake rate (SO₃UR, mgO₃/gCOD.h) was determined as a tool for characterizing the reactivity of the sludges. SO₃UR increased with the X_H fraction and decreased with more X_P. Biomass inactivation was exponential (e^-β.ROD) as a function of the ROD doses. The percentage of solids reduction was predictable through a linear model (C_Miner + Y_sol ROD), with a fixed part due to mineralization (C_Miner) and a variable part from the solubilization process. The parameters of the models, i.e. the inactivation and the dissolution yields (β, 0.008-0.029 (mgO₃/mgCOD_ini)^-1 vs Y_sol, 0.5-2.8 mg COD_sol/mgO₃) varied in magnitude, depending on the intensity of the scavenging reactions and potentially the compactness of the flocs for each sludge.

Bacterial inactivation and regrowth in ozonated activated sludges

Ozonation of the return activated sludge (AS) flow is an emerging option for excess-sludge reduction. This study aimed to evaluate the potential changes suffered by some kinetic parameters of the activated sludge models (ASMs) in the combined ozone-AS process. The heterotrophic maximum specific growth rate (μ_Hmax) was determined by respirometry in three model-sludges (S1 to S3) treated in batch with different O₃ doses. S1 was a fresh synthetic biosolid composed by only two particulate fractions. S2 was a digestate of S1 almost made by the endogenous residues. S3 was from a municipal wastewater treatment plant. μ_Hmax increased significantly from 3.5 d^-1 originally, to more than 10 d^-1 in the ozonated sludges. Ozonation promoted the selection of fast-growing bacteria in the activated sludges, after transitory inactivation and long lag times. Some microorganisms survived to 3 months of digestion and subsequent ozonation, and then regrow faster than before, once fed again with acetate. The research is of interest from the point of view of the application of the ASM models to the ozone-AS process, but also for wastewater disinfection in general.

Microbial community structure of wastewater treatment subjected to high mortality rate due to ozonation of return activated sludge

AIMS

This study investigated the effects of return activated sludge (RAS) ozonation, on the bacterial community structure of pilot-scale wastewater treatment systems.

METHODS AND RESULTS

Two parallel activated sludge reactors were operated to treat real municipal wastewater for 98 days. The RAS of one of the reactors was subjected to increasing doses of ozone during the experimental period, which resulted in higher reduction in biosolids waste production and higher bacterial growth rate. The bacterial community structures were investigated by 16S rRNA gene amplicon high-throughput pyrosequencing and fluorescence in situ hybridization (FISH). The structures remained highly similar throughout the experiment despite the ozone treatment. Comparative analyses between pyrosequencing and FISH revealed clear discrepancies in the proportion of some bacterial populations.

CONCLUSIONS

The results suggest that RAS ozonation is not a main environmental factor structuring the community composition. Instead, the parallel drifts and slight convergence of the two community structures indicate that other environmental factors such as influent wastewater composition and temperature may be more important. Care should be exercised in interpreting the proportion of sequence reads as pyrosequencing may be biased as compared to FISH.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides new insights on the importance of indiscriminate high mortality rates brought by external factors (here ozonation) on microbial community structures of activated sludge system.

New mechanistically based model for predicting reduction of biosolids waste by ozonation of return activated sludge

Two pilot-scale activated sludge reactors were operated for 98 days to provide the necessary data to develop and validate a new mathematical model predicting the reduction of biosolids production by ozonation of the return activated sludge (RAS). Three ozone doses were tested during the study. In addition to the pilot-scale study, laboratory-scale experiments were conducted with mixed liquor suspended solids and with pure cultures to parameterize the biomass inactivation process during exposure to ozone. The experiments revealed that biomass inactivation occurred even at the lowest doses, but that it was not associated with extensive COD solubilization. For validation, the model was used to simulate the temporal dynamics of the pilot-scale operational data. Increasing the description accuracy of the inactivation process improved the precision of the model in predicting the operational data.

Investigation of the impacts of thermal pretreatment on waste activated sludge and development of a pretreatment model

This study investigated the impacts of high pressure thermal hydrolysis (HPTH) pretreatment on the distribution of chemical oxygen demand (COD) species in waste activated sludge (WAS). In the first phase of the project, WAS from a synthetically-fed biological reactor (BR) was fed to an aerobic digester (AD). In the second phase, WAS from the BR was pretreated by HPTH at 150 °C and 3 bars for 30 min prior to being fed to the AD. A range of physical, biochemical and biological properties were regularly measured in each process stream in both phases. The COD of the BR WAS consisted of storage products (XSTO), active heterotrophs (XH) and endogenous decay products (XE). Pretreatment did not increase the extent to which the BR WAS was aerobically digested and hence it was concluded that the unbiodegradable COD fraction, i.e. XE, was unchanged by pretreatment. However, pretreatment did increase the rate of degradation as it converted 36% of XH to readily biodegradable COD (SB) and the remaining XH to slowly biodegradable COD (XB). Furthermore, XSTO was fully converted to SB by pretreatment. Although pretreatment did not change the VSS concentration in the downstream aerobic digester, it did decrease the ISS concentration by 46 ± 11%. This reduced the total mass of solids produced by the digester by 21 ± 8%. A COD-based HPTH pretreatment model was developed and calibrated. When this model was integrated into BioWin 3.1(®), it was able to accurately simulate both the steady state performance of the overall system employed in this study as well as dynamic respirometry results.

Improved solubilization of activated sludge by ozonation in pressure cycles

The generation of a large volume of activated sludge (AS) from wastewater treatment has increasingly become a great burden on the environment. Anaerobic digestion is routinely practiced for excess waste sludge; however, the process retention time is long because of kinetic limitation in the hydrolysis step. We tested the feasibility of applying ozone in pressure cycles to enhance the disintegration and solubilization of AS with the goal to prepare them for digestion using reduced ozone dose and contact time. The AS was subjected to repetitive pressure cycles in a closed vessel in which an ozone gas mixture was compressed into the slurry to reach 1040 kPa in the headspace to be followed by rapid venting. For a returned AS with total COD (tCOD) of 8200 mg L(-1), a dose of 0.01 gO(3)g(-1) total suspended solids (TSS) delivered via 20 pressure cycles within 16 min resulted in a 37-fold increase of the sCOD/tCOD ratio (due to increased soluble COD, i.e. sCOD) and a 25% reduction of TSS, in comparison to a dose of 0.08 gO(3)g(-1) TSS via bubbling contact over 15 min that resulted in a 15-fold increase of the sCOD/tCOD ratio and a 12% reduction of TSS. Sludge solubilization was evidenced by increased dissolved contents of total phosphorous (from 10 to 64 mg L(-1)), total nitrogen (from 14 to 120 mg L(-1)), and protein (from <15 to 39 mg L(-1)) in the sludge suspension after treatment, indicating significant solubilization of AS.