Water Journal : Water Journal May 2011
biosolids refereed paper technical features 88 MAY 2011 water This cascade effect stops the raw sludge withdrawal for around 25 minutes in every hour, restricting the capacity to remove solids from the influent. Therefore, to increase the mass of raw sludge withdrawal, the volume fed to the digesters each hour must be increased or the sludge further thickened. Thickening the sludge to 6% TSR was considered the limit to mitigate high pressure pumping and impacts on digester mixing. Monitoring of raw sludge withdrawal volumes over the recuperative thickening trial has demonstrated the ability of this process to increase raw sludge withdrawal (see Figure 3). Recuperative thickening accounts for increases of 2% to 26% in daily raw sludge withdrawal, depending on hours of operation. By running the process continuously for 20 hours on 17/7/10, a 26% increase in raw sludge withdrawal was observed (54.3 dT/d, as opposed to the 43.0 dT/d predicted had recuperative thickening not run). Digester performance Table 2 shows how the digesters have been operated differently from the design specifications. To process the increased solids inventory in the PSTs, the digester feed rate was increased. This increased digester feed rate has led to reduced SRT in the digesters. With the implementation of recuperative thickening, the removal of excess water from the digesters has effectively increased digester capacity. Figure 4 shows the effective change in digester capacity as a result of running the process. Operational flexibility The above discussion of raw sludge withdrawal and digester performance illustrates that without recuperative thickening, the operating range for digester feed is restricted. Assuming a minimum SRT of 15.8 days this operating range is only 14.5--16m3/h per digester. Increasing this set point would lead to very low HRT/SRT in the digester. Conversely, reducing this set point would increase HRT/SRT but would further throttle the hydraulic capacity of raw sludge withdrawn, which would likely result in high effluent suspended solids carryover and possible breach of licence conditions. Recuperative thickening was trialled as a way to increase the available capacity of the digesters to provide in excess of the required 20 days SRT, thus providing process flexibility. To determine process implications a mass balance was prepared with the following operating scenarios: • Design model: Raw sludge withdrawal as predicted during plant design; • Current practise: the way the plant is actually run without recuperative thickening; • Maximise raw sludge withdrawal: running recuperative thickening so that SRT is unchanged from current practise; • Maximise digestion: running recuperative thickening so that raw sludge withdrawal is unchanged from current practise. Some of the parameters from the mass balance are presented in Table 3 (see overleaf). In the "maximise raw sludge withdrawal" model, recuperative thickening removed excess water from the digesters creating headspace within the digester. This headspace allows higher digester feed rates, which has the flow-on effect of larger volumes of raw sludge to be withdrawn from PSTs. During the trial it was observed that the raw sludge pumps stop due to high downstream tank level only four minutes every hour, instead of 25 minutes an hour with no recuperative thickening. This reduces the sludge inventory in the PST's sludge hopper, reducing the risk of sludge rafting and solids passing through to effluent. This benefit was seen in a gradual reduction in raw sludge TSR over a few successive days of operation. In the "maximise digestion" scenario, recuperative thickening improved digestion by increasing digester SRT. This increases the solids concentration within the digester and reduces the hydraulic volume leaving the digesters to dewatering. This reduced hydraulic load has a significant advantage of enabling the throughput of the centrifuge to draw down the post-digestion tank level, thus mitigating overflows and solids recycling. Because the digester TSR increases, so too does the TSR of the dewatering feed. The combination of reduced hydraulic load and increased centrifuge feed solids is expected to lead to reduced centrifuge operating time and improved operation. Manual operation of recuperative thickening for just seven hours per day reduced the sludge volume transferred from digester to dewatering and stopped the post-digestion tank overflow for up 10 hours. The centrifuge normally runs 24 hours a day, however, while recuperative thickening the centrifuge shut down on low tank level for up to 6.5 hours, a saving on both energy and polymer costs. After two weeks of daily recuperative thickening operation, one less biosolids truck was needed. This could represent significant long-term cost savings and environmental benefits. With recuperative thickening, the digester feed operating set point is no longer restricted to a batch feed of 14.5--16m3/h/dig (min. 15.8 days SRT=HRT). Now it can be increased to 23-29m3/h/dig (SRT≠HRT). A wider operating range means the plant will be better equipped to cope with changes in solids inventory and digester performance. Table 2: Digester parameters. Parameters Design Actual Feed solids content 6.0% 5.5% Volumetric feed rate (m3/h per digester) 12.4 16.0 SRT (days) 20.0 15.8 0.0 10.0 20.0 30.0 40.0 50.0 60.0 6/30/2010 7/2/2010 7/4/2010 7/6/2010 7/8/2010 7/10/2010 7/12/2010 7/14/2010 7/16/2010 Date Raw sludge withdrawal (dT/day) Extra raw sludge withdrawn as a result of recuperative thickening Raw sludge withdrawal had recuperative thickening not run Figure 3: Raw sludge withdrawal. 13000 13100 13200 13300 13400 13500 13600 13700 30/06/10 2/07/10 4/07/10 6/07/10 8/07/10 10/07/10 12/07/10 14/07/10 16/07/10 Date Total digester capacity (m 3) Extra capacity generated by recuperative thickening Digester capacity without recuperative thickening Figure 4: Digester capacity.
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