The conventional mainstream enhanced biological phosphorus removal (EBPR) process depends on the quality of the raw incoming wastewater. An alternative sidestream EBPR process is presented, where the substrates for storage by the polyphosphate accumulating organisms (PAOs) instead come from hydrolysis of the return activated sludge. This process is studied in full-scale at two treatment plants and quantified by means of phosphorus release rates and readily biodegradable COD (RBCOD) accumulation rates. It was seen that not only was a significant amount of RBCOD stored by PAOs but an approximately equal amount was accumulated in the sidestream hydrolysis tank and made available for the subsequent nitrogen removal process. The phosphorus release of the sludge with and without addition of different substrates was furthermore studied in laboratory scale. The study showed that the process is promising and in a number of cases will have significant advantages compared with the conventional mainstream EBPR
Increasing sludge disposal costs have highly intensified the interest in reducing the sludge quantities from Danish wastewater treatment plants. By upgrading existing mesophilic digesters to the thermophilic temperature range, the retention time can be halved and many digesters designed only for primary sludge will have sufficient capacity to treat also the biological excess sludge. At the moment, eight full-scale thermophilic digesters are in operation in Denmark and five are under construction. This paper describes the full-scale experience gained from digestion of biological excess sludge as well as a mixture of primary and biological sludge. Thermophilic digestion has proven to be a good and stable process for solids reduction and pathogen removal. The digested sludge can be dewatered to a high solids content and thereby the sludge quantity for disposal can be reduced by 30-40% depending on the type of wastewater treatment plant. A drawback of the process is that the polymer costs for sludge dewatering may be increased depending on the sludge type.
Biological sludge hydrolysis was demonstrated in full scale at three Danish wastewater treatment plants. For primary sludge the hydrolysis yield expressed in terms of filtrable COD varied from 9–16% of the total COD in the sludge (WTPs 1 & 2) and for the hydrolysis of activated sludge a yield of 2.5% of the total COD was found. The addition of hydrolysate was demonstrated to improve the biological P removal considerably. No effect on the nitrogen removal could be identified due to a favourable wastewater composition during the demonstration phase. A cost-benefit analysis showed that biological sludge hydrolysis may be a cost efficient process that should be considered in connection with the upgrading of wastewater treatment plants to nutrient removal.
Anaerobic hydrolysis in activated return sludge was investigated in laboratory scale experiments to find if intermittent aeration would accelerate anaerobic hydrolysis rates compared to anaerobic hydrolysis rates under strict anaerobic conditions. The intermittent reactors were set up in a 240 h experiment with intermittent aeration (3h:3h) in a period of 24 h followed by a subsequent anaerobic period of 24 h in a cycle of 48 hours which was repeated 5 times during the experiment. The anaerobic reactors were kept under strict anaerobic conditions in the same period (240 h). Two methods for calculating hydrolysis rates based on soluble COD were compared. Two-way ANOVA with the Bonferroni post-test was performed in order to register any significant difference between reactors with intermittent aeration and strictly anaerobic conditions respectively. The experiment demonstrated a statistically significant difference in favor of the reactors with intermittent aeration showing a tendency towards accelerated anaerobic hydrolysis rates due to application of intermittent aeration. The conclusion of the work is thus that intermittent aeration applied in the activated return sludge process (ARP) can improve the treatment capacity further in full scale applications.
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