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The MBR has generally been operated at a total SRT, based on the inventory in the bioreactor, of 15 to 16 days, and the mixed liquor suspended solids concentration was generally in the range of 5,000 to 6,000 mg/L (varying with influent loading conditions of course). It is also worth noting that the influent temperature varies over the year from a low in winter of 15 °C to 16 °C to a high in late summer of 22 °C to almost 23 °C. The aerobic SRT was generally 7 to 8 days. Table 4 summarizes average effluent quality for the intensive evaluation period, which indicates excellent performance. Effluent total and soluble chemical oxygen demand (COD) values are essentially the same, as expected. Effective nitrification along with removal of total nitrogen (TN) and total phosphorus (TP) was achieved. Domestic MBR effluent nutrient concentrations were variable, however, as illustrated in Figures 2 and 3 where effluent TN, ammonia, and TP results are presented for the entire operating period (2010 through 2014). Effluent TN concentrations were generally around 5 mg/L or below for most of the data period, but with occasional elevated values. Increased effluent TN was generally a result of increased effluent ammonia. Effluent ammonia and TN concentrations became more stable and declined, beginning the second half of 2013 as a result of ongoing process optimization efforts during this time period. These efforts were focused primarily on improved DO control in the MBR bioreactor. Effluent TP was variable throughout the entire period, except for two periods of more stable performance in late 2013 and early 2014 (Figure 3). Analysis of operating data during 2013 and early 2014 confirmed that improved effluent TN, and the period of improved effluent TP, were a result of improved DO control (CH2M HILL 2014). This work, and principal finding, led to a program of improvements targeting DO control. Actions, to date, have addressed balance tank level management to reduce influent stoppages (when the balance tank reserve was exhausted), optimized positioning of the oxidation-reduction potential (ORP) probe in anoxic zones, moderate reduction in internal recirculation to reduce DO return to the anoxic zone, adjustment of DO set points and aeration control loop tuning. However, although some improvement in MBR effluent quality has been reported so far, improvement work continues, and the desired results are yet to be fully demonstrated. The impact of these variations of MBR nutrient removal on RO performance is addressed below.
Table 4

Domestic MBR effluent quality, December 2010 through October 2012

ItemMBR influentMBR effluentRemoval (%)
COD (mg/L) 422 35.4 92 
sCOD (mg/L) 255 35.4 86 
TN (mg-N/L) 35.6 3.6 87 
NH4-N (mg-N/L) 26.6 1.4 95 
TP (mg-P/L) 8.2 2.8 66 
ItemMBR influentMBR effluentRemoval (%)
COD (mg/L) 422 35.4 92 
sCOD (mg/L) 255 35.4 86 
TN (mg-N/L) 35.6 3.6 87 
NH4-N (mg-N/L) 26.6 1.4 95 
TP (mg-P/L) 8.2 2.8 66 
Figure 2

Domestic MBR effluent TN and ammonia concentration for 2010 through 2014.

Figure 2

Domestic MBR effluent TN and ammonia concentration for 2010 through 2014.

Figure 3

Domestic MBR effluent TP for 2010 through 2014.

Figure 3

Domestic MBR effluent TP for 2010 through 2014.

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