各种污水生化处理工艺流程框图

各种工艺流程框图

The MLE consists of an anoxic basin upstream of an aerobic zone. An internal recycle carries nitrates created during the nitrification process in the aerobic zone along with mixed liquor to the anoxic zone for denitrification. RAS is mixed with the influent to the anoxic zone. The extent of denitrification is tied to the mixed liquor recycle flow; higher recycle rates increase denitrification. Because only recycled nitrate has the opportunity to be denitrified, the MLE alone cannot achieve extremely

low final nitrogen concentrations. The maximum denitrification potential is approximately 82 percent at a 500 percent recycle rate (WEF and ASCE 2006). TN effluent concentrations typically range from 5 to 8 mg/L (Barnard 2006). Actual denitrification might be limited by other factors, such as carbon source availability, process kinetics, and anoxic or aerobic zone sizes. Furthermore, oxygen recycled from the aerobic zone can negatively affect the denitrification rate in the anoxic zone (WEF and ASCE 2006). Performance factors include limitations due to the single anoxic zone and the internal recycle rate that returns nitrates to the anoxic zone. Selection factors include the possibility of constructing walls in existing basins to create an anoxic zone; additional pumping, piping, and electricity to accommodate the internal recycle; and the possible need for an additional carbon source to promote denitrification. The modified Ludzack-Ettinger process is illustrated in Figure 2-3.

IFAS systems are hybrids that have attached growth media included in an activated-sludge basin.

Schreiber Process

The patented Schreiber countercurrent aeration process can provide nitrification and denitrification in one basin. The wastewater enters a circular basin equipped with a rotating bridge that provides mixing. Aeration is provided by fine-bubble diffusers attached to the bridge. Should sequencing between aerobic and anoxic conditions be required, the aeration can be turned off while the bridge continues to keep the tank mixed. The system footprint is approximately that of a conventional activated-sludge system but with additional equipment built in. Because it includes alternating aerobic/anoxic conditions in one tank, very low effluent TN concentrations are possible, when optimized. The performance could be further enhanced with additional anoxic zones downstream of the primary reactor. In the standard Schreiber configuration, the need for additional food is typically reduced or eliminated because everything is done in one tank. The Schreiber countercurrent aeration process is illustrated in Figure 2-12. The TN effluent concentrations for the Schreiber process in the literature evaluated as part of this study was 8.0 mg/L (see Table 2-1 in Section 2.5).

The Virginia Initiative process (VIP) is similar to the modified UCT process and is another variation of the Phoredox process. The nitrates from the aerobic zone are returned to the head of the first anoxic zone, instead of the second anoxic zone as is done with the modified UCT process. The second return is from the end of the second anoxic zone to the head of the anaerobic zone. RAS continues to enter the head of the first anoxic zone. The VIP process allows for additional denitrification and thus minimizes the introduction of nitrate to the anaerobic zone. Nitrate in the anaerobic zone would interfere with phosphorus release, and so would reduce the opportunity for subsequent phosphorus uptake in the aerobic zone. The VIP process is operated in a high-rate mode, allowing for small tank volumes, which require less space than other similar

processes. This process has a medium-sized footprint and could be set up in existing basins. As with other processes in this section, no additional head is needed, but extensive piping and pumping are needed for the recycle streams. As with the other processes, if sufficient VFAs are present, no supplemental carbon sources are required. Achieving a very low phosphate concentration requires downstream chemical precipitation and filtration. The VIP is illustrated in Figure 2-26. The ranges of TN and phosphorus effluent concentrations found in the literature for the VIP were 3.0 mg/L to 10.0 mg/L in TN and 0.19 mg/L to 5.75 mg/L in TP; for the VIP with VFA addition the ranges were 5.0 mg/L to 10.0 mg/L in TN and 0.6 mg/L to 0.8 mg/L in TP (see Tables 2-1, 2-2, and 2-3 in Section 2.5 for details).

Blue Plains Process

The Blue Plains process was a retrofit to the existing nitrification activated-sludge process at the Washington, DC, facility. A new anoxic zone was created inside the aeration tank with an HRT of 0.8 hour from the nominal 3.3 hours in the total basin. The design sludge age was 13 days. The existing return activated-sludge system remained unchanged in this retrofit. Methanol was fed directly into this new anoxic zone for a target nitrogen concentration of 7.5 mg/L (Kang et al. 1992; Sadick et al. 1998). Phosphorus is removed by ferric chloride addition and tertiary filtration. The Blue Plains process is depicted in Figure 2-29. The TN and TP effluent concentrations found in the literature for the Blue Plains process were 7.5 mg/L in TN and 0.12 mg/L in TP (see Table 2-1 in Section 2.5 for details).