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Two different types of biomass, capable for Enhanced Biological Phosphorus Removal (EBPR), a UCT (University of Cape Town) type and a sludge enriched with DPAOs (Denitrifying Phosphorus Accumulating Organisms) were tested in batch reactors under specific operational and environmental conditions, in order to achieve a direct comparison of their phosphorus removal capability. Three types of batch reactors were operated, Anaerobic/Oxic (AO), Anaerobic/Anoxic (A2) and Anaerobic/Anoxic/Oxic (A2O), under controlled temperature and pH conditions. Maximum anaerobic specific phosphate release, substrate utilization, as well as denitrification and phosphate uptake rates under aerobic and anoxic conditions were determined and compared for the two different microbial populations. Experimental results indicated no significant difference between the anoxic and the aerobic phosphorus (P) uptake rates, respectively for DPAO and UCT sludge. The UCT sludge was also found to achieve anoxic P uptake, however to much less extend compared to the DPAO sludge. It has also been proved that anoxic P uptake seems to negatively affect the total P removal efficiency of this type of sludge, even under following aerobic conditions. Based on these findings, denitrifying phosphorus removal systems are proved comparable to conventional EBPR configurations (UCT), concerning phosphorus removal efficiency, while their operation is accompanied by potential advantages.

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A continuous-flow anaerobic-anoxic (A2) activated sludge system was operated for efficient enhanced biological phosphorus removal (EBPR). Because of the system configuration with no aeration zones, phosphorus (P) uptake takes place solely under anoxic conditions with simultaneous denitrification. Basic operating conditions, namely biomass concentration, influent carbon to phosphorus ratio and anaerobic retention time were chosen as variables in order to assess their impact on the system performance. The experimental results indicated that maintenance of biomass concentration above 2,500 mg MLVSS/L resulted in the complete phosphate removal from the influent (i.e. 15 mg PO(4) (3-)-P/L) for a mean hydraulic residence time (HRT) of 15 h. Additionally, by increasing the influent COD/P ratio from 10 to 20 g/g, the system P removal efficiency was improved although the experimental results indicated a possible enhancement of the competition between phosphorus accumulating organisms (PAOs) and other microbial populations without phosphorus uptake ability. Moreover, because of the use of acetate (i.e. easily biodegradable substrate) as the sole carbon source in the system feed, application of anaerobic retention times greater than 2 h resulted in no significant release of additional P in the anaerobic zone and no further amelioration of the system P removal efficiency. The application of anoxic P removal resulted in more than 50% reduction of the organic carbon necessitated for nitrogen and phosphorus removal when compared to a conventional EBPR system incorporating aerobic phosphorus removal.

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Biohydrogen production from a simulated fruit wastewater (soluble COD = 3.17 ± 0.10 g L⁻¹) was carried out in a continuous stirred tank reactor (CSTR) of 2 L operational volume without biomass inoculation, heat pre-treatment or pH adjustment, resulting in a low operational pH (3.75 ± 0.09). The hydraulic retention time (HRT) varied from 15 to 5 h. A strong negative correlation (p < 0.01) between the biogas production rate and the HRT was observed. Biogas production rates were higher at 30 °C than at 25 °C (p < 0.01), when the CSTR was operated under the same HRT. The biogas hydrogen content was estimated as high as 55.8 ± 2.3 % and 55.4 ± 2.5 % at 25 and 30 °C, respectively. The main fermentation end products were acetic and butyric acids, followed by ethanol. Significant differences (p < 0.01) during the operation of the CSTR at 25 or 30 °C were identified for butyric acid at almost all HRTs examined. Simulation of the acidogenesis process in the CSTR (based on COD and carbon balances) indicated the possible metabolic compounds produced at 25 and 30 °C reactions and provided an adequate fit of the experimental data.