Maximisation of the organic load rate and minimisation of oxygen consumption in aerobic biological wastewater treatment processes by manipulation of the hydraulic and solids residence time

Abstract

5A systematic experimental study of the effect of hydraulic residence time (HRT) and 6 solids residence time (SRT) on conventional suspended-growth biological wastewater 7 treatment processes was carried out. The aim of this study was to identify the 8 conditions that minimise the reactor volume, i.e. maximise the organic load rate (OLR), 9 and minimise the oxygen consumption. Lab-scale sequencing batch reactors (SBRs) 10 were operated with glucose or ethanol as only carbon sources, with HRT in the range 11 0.25-4 day and SRT in the range 1-71 day. The highest OLR values which gave 12 satisfactory performance were 4.28 and 4.14 gCOD/l.day for glucose and ethanol, 13 respectively, which are among the highest reported for conventional aerobic 14 suspended-growth processes. The highest OLR values were obtained with HRT=0.25 15 day, SRT=3.1 day for glucose and HRT=0.5 day, SRT=4.9 day for ethanol. The minimum 16 oxygen consumption was 0.36 and 0.69 kg O2/kg COD removed for glucose and 17 ethanol, respectively. In disagreement with conventional theories, it was found that 18 biomass production also depended on the OLR as well as on the SRT, higher OLRs 19 giving lower biomass production for the same SRT. From the kinetic analysis of the 20 experimental data, this behaviour, which has important consequences for the design 21 of biological wastewater treatment processes, was explained with a higher rate of 22 endogenous metabolism at higher OLRs. 23The aim of aerobic biological wastewater treatment processes is to treat the influent 30 wastewater with the highest possible reduction of the COD and BOD, with the 31 minimum possible size of the reaction tank and the minimum possible oxygen 32 consumption. A high COD reduction is required to maintain the high environmental 33 quality of the receiving water body, a small volume of the reaction tank decreases the 34 capital costs and the land usage by the plant, low oxygen consumption minimises the 35 energy costs and the environmental footprint of the plant. In addition, the production 36 of waste sludge needs also to be taken into account in the design of biological 37 treatment processes. Usually, waste sludge is considered a liability which needs to be 38 minimised, but the increasing use of anaerobic digestion to convert sludge into 39 methane is showing that waste sludge can rather be seen as a resource (McCarty et al., 40 2011). 41As far as the reactor volume is concerned, for a given flow rate and composition of the 42 influent wastewater, smaller reactor volumes correspond to lower values of the 43 hydraulic residence time (HRT) and, correspondingly, to higher values of the 44 volumetric organic load rate (OLR). In conventional suspended-growth activated 45 sludge processes, the OLR is typically in the range 0.5-1.5 kg COD/m 3 .day (WEF, 2012). 46 Various technologies have been investigated to increase the OLR and therefore 47 decrease the reactor volume, e.g. air-bubble or jet-loop bioreactors, membrane 48 bioreactors or granular sludge. For example, Petruc...

Effect of process conditions on the aerobic biodegradation of phenol and paracetamol by open mixed microbial cultures

“…In both cases nutrients the mineral medium (prepared with tap water) also included: NH4Cl (0.8 g/l), K2HPO4 (3.5 g/l), NaH2PO4 (2.4 g/l), thiourea (20 mg/l). The mineral medium composition was in line with the one used in our previous work [12]. In some batch experiments, as described later, a mixture of peptone and yeast extract (0.1 g/L each, total 0.2 g/L) was also added.…”

“…The inoculum was a soil from Craibstone farm in Aberdeen, Scotland (0.1 gVSS/g soil [12]). The soil was homogenised and sieved (150 mm size) and then stored in plastic containers at room temperature before inoculation.…”

“…The reactors were glass vessels with a working volume of 0.25L for the batch and 1L for the SBRs and the set-up was similar as in our previous study [12]. VELP SCIENTIFIC ® magnetic multi stirrer with 15 stirring positions was used to provide mixing for the reactors (magnetic stirrer) for the batch.…”

“…In the other runs the SRT was higher than the HRT, the sludge withdrawal was done manually once per day at the end of the React phase and the appropriate volume of clarified effluent was collected by a pump at the end of the Settle phase. This procedure was the same as in our other recent paper [12]. In the runs with SRT>HRT the average SRT was calculated at the end of each run from the average steady-state concentrations of solids in the well-mixed reactor and in the effluent according to equation (1) [12]:…”

“…This procedure was the same as in our other recent paper [12]. In the runs with SRT>HRT the average SRT was calculated at the end of each run from the average steady-state concentrations of solids in the well-mixed reactor and in the effluent according to equation (1) [12]:…”