NC0047384_Application (ATC) Calculations_20200108

Hazen and Sawyer
T. Z. Osborne WRF BNR Upgrade Project – Package 4 – Blower Calculations
Authorization to Construct Application: Design Calculations
Design Calculations Table of Contents
Tab 1. Aeration Demand
Tab 2. Sizing Criteria for Aeration Blowers
Hazen and Sawyer
T. Z. Osborne WRF BNR Upgrade Project – Package 4 – Blower Calculations
Authorization to Construct Application: Design Calculations
1. Aeration Demand
The expansion of the T.Z. Osborne WRF to 56 mgd and conversion to 5-stage BNR will require
the installation of new diffusers. Calculations for determining process air demands and for
determining the number of fine bubble diffusers required are summarized in Table 1.

Table 1 Process Aeration Requirements
Parameter
Current Annual
Average 2013-
2015
Design
Annual
Average
Design
Maximum
Month
Design
Maximum
Day
Flow, MGD 26.4 48.6 57.2 141.2
Number of Equivalent Basins in Service1 12 20 20 22
Influent Flow per Basin, mgd 2.00 2.43 2.86 6.42
Primary Effluent BOD5, mg/L 333 294 323 299
Primary Effluent TKN, mg/L 35.6 33.6 34.2 26.2
Effluent Nitrate, mg/L 8 3 3 12
Actual O2 Requirement (AOR), lb/day 101,000 162,000 207,000 494,000
Alpha 0.64 0.64 0.64 0.64
Temperature oC 23 23 29 30
DO Concentration, mg/L 2.0 2.0 2.0 0.5
AOR/SOR 0.47 0.47 0.47 0.58
Standard Oxygen Requirement, lb
O2/day 214,000 344,000 441,000 581,000
Standard Oxygen Transfer Efficiency, % 37.0 37.1 36.7 33.4
Number of Diffusers 16,608 27,680 31,8602 35,0462
Airflow per Diffuser, scfm 1.40 1.34 1.51 2.91
Airflow at Design Efficiency, scfm 23,100 36,900 48,000 102,000
1 Basins 1-4 are twice as large as other basins and are considered two “equivalent basins”
2 Assumes the first cell in the post-anoxic zone is in service
In addition to providing process air to the aerobic BNR cells, the aeration system must provide
sufficient air to the aeration basin reaeration zones and effluent channel to keep solids in
suspension and mixed. Reaeration zone air requirements are based on providing sufficient
aeration to provide an airflow rate of 0.12 scfm per square foot of surface area within the zone.
The required air flow rate for reaeration zone mixing is approximately 800 scfm. Effluent
channel air requirements are based on providing an airflow rate of 20 cfm per 1,000 cubic feet
of channel volume to maintain solids in suspension. The air flow rate for channel mixing is 1,750
scfm based on an expanded effluent channel volume of 0.65 MG. The resulting required peak
day aeration capacity is 104,550 scfm (117,450 icfm) and includes the process aeration demand
for design max day loads, as well as the mixing requirements assuming all 18 basins and the
entire effluent channel in operation.
Hazen and Sawyer
T. Z. Osborne WRF BNR Upgrade Project – Package 4 – Blower Calculations
Authorization to Construct Application: Design Calculations
2. Sizing Criteria for Aeration Blowers
Table 1 summarizes the horsepower and capacities (at 10.5 psig discharge pressure) of the
existing blowers. The total system capacity is 88,500 icfm.

Table 1: Current Blower Capacity
Blower Type Horsepower
(HP)
Capacity (icfm)
1 Multi-Stage 1,750 22,600
2 Multi-Stage 1,750 22,600
3 Single-Stage 1,300 21,650
4 Single-Stage 1,300 21,650
TOTAL 88,500

Table 2 summarizes the horsepower and capacity of the blower system after Multi-Stage blower
number 2 is replaced with a new, larger single-stage blower during the Package 4 upgrade. The
total system capacity will be 102,900 icfm.

The 56 MGD maximum day aeration demand for calculated in Section 1 was 117,300 ICFM.
This aeration demand includes influent loading from many industrial users and represents a
worst-case scenario from an aeration perspective. There are times when influent load
decreases by 20 to 30 percent when the industry is not discharging. Due to the high variability of
influent loading and the uncertainty surrounding future industrial discharge load it was decided
to install one new single-stage blower now and install one new single stage blower in the future
if the industrial loadings continue to be high.

An evaluation determined that Multi-Stage Blower number 1 would need to be replaced by 2036
or 2027 (assuming 1% or 2% population growth, respectively) to achieve the full blower system
capacity of 117,300 ICFM.

Table 2: Blower Capacity after Package 4
Blower Type Horsepower
(HP)
Capacity (icfm)
1 Multi-Stage 1,750 22,600
2 New Single-Stage 2,000 37,100
3 Single-Stage 1,300 21,650
4 Single-Stage 1,300 21,650
TOTAL 103,000

Table 3 summarizes the horsepower and capacity of the blower system after Multi-Stage blower
number 1 is replaced with a new, larger single-stage blower after the Package 4 upgrade. The
total system capacity will be 117,500 icfm.

Hazen and Sawyer
T. Z. Osborne WRF BNR Upgrade Project – Package 4 – Blower Calculations
Authorization to Construct Application: Design Calculations
Table 3: Future Blower Capacity
Blower Type Horsepower
(HP)
Capacity (icfm)
1 New Single-Stage 2,000 37,100
2 Single-Stage 2,000 37,100
3 Single-Stage 1,300 21,650
4 Single-Stage 1,300 21,650
TOTAL 117,500