Water and Wastewater Engineering - Sciences Club

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-79
Temperature. Very good BPR performance can be achieved as long as SRT values of 16 and
12 days are provided for temperatures of 5 � C and 10 � C, respectively. SRTs between 16 and
24 days did not affect system performance at 5 � C. At 10 � C, SRTs between 12 and 17 days did not
affect system performance (WEF, 2006a).
Recycle streams. Solids processing return flows from sludge thickening, anaerobic digestion,
and sludge dewatering typically contain high ammonia and phosphorus concentrations. The high
concentrations and variable rates of generation will degrade the performance of BPR process if
they are added on an ad hoc basis. At a minimum, the rate of flow should be equalized. Adding
these streams when the strength of the influent wastewater is stronger (typically, during daytime)
will help to increase the removal of recycled phosphorus. Separate treatment with chemical
addition is the preferred alternative (Metcalf & Eddy, 2003; WEF, 2006a).
Chemical addition. To achieve phosphorus concentrations below 1.0 mg/L as well as to provide
backup for the BPR process, chemical addition facilities should be provided.
Design Practice for Denitrification. The following paragraphs outline the design practice for
those portions of the BPR process that affect denitrification and its relationship to phosphorus
removal. The discussion is focused on the A 2 /O process. Alternative processes will require
modifications of the design practices noted here. These are discussed in Metcalf & Eddy (2003),
WEF (1998), and WEF (2006a).
Hydraulic residence time (HRT). The anoxic tank is typically sized based on the amount of
nitrate to be denitrified. As discussed in the previous paragraphs on SBR design practice, two
design approaches are used to check the anoxic HRT. The method used here is a desktop approach
that uses mass balances for nitrogen and the specific denitrification rate (Equation 23-52).
Simulation modeling is the alternative approach.
The desk top method is iterative. The NO x formed in nitrification is estimated. The assumed
anoxic HRT and SDNR are used to estimate the amount of NO x that can be denitrified in the fill
time. If it is greater than the NO x formed, the fill time is acceptable. If it is not, another iteration
with a new anoxic fill time is performed.
The major difference between the computational procedure here and that used for the SBR is
that internal recycle (Equations 23-54 through 23-57) must be considered.
T ypical HRTs for the A 2 /O process vary between 0.5 and 1.5 hours. Typical HRTs for
other processes are given in Table 23-9 .
Internal recycle. Equation 23-54 is used to estimate the internal recycle ratio (IR). To meet a
standard of 10 mg/L total nitrogen or less, the design effluent NO 3 -N concentration should be in
the range of 5 to 7 mg/L.
The nitrate concentration in the RAS flow can have a significant adverse effect on the amount
of rbCOD that is available for BPR. The nitrate consumption of rbCOD can be estimated from
the ratio of rbCOD/NO 3 -N used. It is 6.6 g rbCOD/g NO 3 -N (Metcalf & Eddy, 2003).
An internal recycle ratio in the range of 3 to 4 is typical, but ratios in the range of 2 to 3
are also used with lower influent wastewater TKN concentrations. Recycle ratios above 4 are
generally not warranted because the incremental NO 3 -N removal is low and more DO is recycled
to the anoxic tank (Metcalf & Eddy, 2003).