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Chemical Addition Overview
Purpose
The wastewater treatment chemical systems are capable of delivering (1) sodium hypochlorite to the non-potable water system for disinfection of the plant water system, (2) sodium hypochlorite to the RAS Wet Well for intermittent application to control growth of filamentous organisms, (3) magnesium hydroxide to boost alkalinity in the aeration basin influent, (4) alum to improve solids settling, and (5) polymer to improve solids settling. Sodium hypochlorite is stored in the Chlorine Building, while the other chemicals are located in the Chemical Room at the north end of the Blower Building. A counteractant system is provided at the Equalization Basin to reduce potential impacts from odorous air.
Description
Sodium Hypochlorite System
The sodium hypochlorite system consists one storage tank and three feed pumps. Also included in the pumping system are calibration columns, pulsation dampeners, pressure relief valves, and diaphragm valves. See Figure 2 for system details. The tank is rated for a capacity of 1000 gallons. The tank is filled by chemical delivery trucks which connect to fill connection located at the tank. Sodium hypochlorite flows by gravity to three feed pumps. The sodium hypochlorite is then pumped from the feed pumps to the appropriate dosage location.
Magnesium Hydroxide System
The magnesium hydroxide system consists of a 5,000 gallon XLPE (cross-linked polyethylene) storage tank, gear drive tank mixer mounted on a mixer bridge, and a peristaltic (tubing) chemical feed pump. Magnesium hydroxide is delivered to an injection point in the Aeration Basin Inlet Channel. The chemical feed rate is based on the operator-input dosage and is automatically flow-paced to plant effluent flow.
Alum System
The alum system consists of a 5,000 gallon XLPE storage tank and a motor-driven, air-diaphragm chemical feed pump. Alum is delivered to two injection points: the Secondary Clarifier Splitter Box and the Pre-aeration Tank. The chemical feed rate is based on the operator-input dosage and is automatically flow-paced to plant effluent flow.
Polymer System
The polymer system consists of a 2,500 gallon XLPE storage tank, gear drive tank mixer mounted on a mixer bridge, dry polymer eductor, dilution/mixing water system, and a progressing cavity polymer feed pump. Polymer is delivered to two injection points: the Secondary Clarifier Splitter Box and the Pre-aeration Tank. The polymer feed rate is based on the operator-input dosage and is automatically flow-paced to plant effluent flow.
Counteractant System
The counteractant system consists of chemical storage drum, high-pressure injection pump system, duplex chemical feed pumps, and dilution water system mounted in a heated FRP enclosure. An associated backflow preventer for the water supply is installed within a heated enclosure on the same concrete pad next to the counteractant system. Counteractant chemical is dispersed into the atmosphere through injector nozzles installed along high-pressure tubing around the perimeter of the Equalization Basin. The nozzle system is divided into north and south halves around the Basin and can be operated independently or in tandem as conditions dictate.
Operation
Sodium Hypochlorite System
Calculations
The equation for calculating sodium hypochlorite usage is:
Usage = Dose * Flow * 8.34
Where:
Usage = Pounds per day of sodium hypochlorite (ppd)
Dose = Sodium hypochlorite dose (mg/L or ppm)
Flow = Plant water or RAS flow (mgd)
8.34 = Conversion factor (lb/gal)

An example of a sodium hypochlorite usage calculation with a flow of 5.5 mgd is:
Given:
Dose = 1.0 ppm (mg/L)

Usage = 1.0 mg/L * 5.5 mgd * 8.34 lb/gal
Usage = 46 lb/day

All hypochlorite calculations should reflect the actual activity of the sodium hypochlorite. Sodium hypochlorite degrades quickly in elevated temperatures, and with exposure to UV light. An activity factor (i.e., the solution is less than the stated 12.5% strength when purchased) should be considered when dosing aged hypochlorite.
Magnesium Hydroxide System
Feed Rate
Biological treatment of municipal wastewater typically results in reduction in alkalinity. In order to promote biological treatment and replace lost alkalinity, magnesium hydroxide is often added. Wenatchee WWTP has typically added 1,000 to 2,000 lbs/day of dolomite for a plant flow of 3.3 mgd and obtained beneficial results from increased alkalinity and/or magnesium.

Magnesium hydroxide is metered into the Aeration Basin Influent at a dosage that will provide approximately 80 mg/L alkalinity as CaCO3 at the maximum month flow rate of 5.5 mgd. The capacity of the chemical feed pump is flow paced to automatically maintain the dosage as the plant flow increases and decreases throughout the day. Effluent plant flow is taken as a surrogate for influent flow and the dosage adjusted accordingly to optimize chemical and/or treatment performance.
Calculations
The PICS system automatically adjusts pump speed in response to plant effluent flow and based on chemical dosage and pump delivery (pump capacity x chemical concentration) as follows:

Pump Speed (%) = [Effluent Flow (mgd) x Dosage (mg/L) x C]/Pump Delivery (lbs/day)

Where:
C = Units Constant of 34.78 to calculate pump speed in %
Dosage = Desired magnesium hydroxide dose (typically 46 mg/L)
Pump Delivery = Maximum Pump Capacity (11.4 gal/hr) x Chemical Concentration (7.8 lbs/gal)
Alum System
Calculations
The PICS system automatically adjusts pump speed in response to plant effluent flow and based on chemical dosage and pump delivery (pump capacity x chemical concentration) as follows:

Pump Speed (%) = [Effluent Flow (mgd) x Dosage (mg/L) x C]/Pump Delivery (lbs/day)

Where:
C = Units Constant of 34.78 to calculate pump speed in %
Dosage = Desired alum dose (typically 55 mg/L)
Pump Delivery = Maximum Pump Capacity (19.2 gal/hr) x Chemical Concentration (5.4 lbs/gal)
Polymer System
Calculations
The PICS system automatically adjusts pump speed in response to plant effluent flow and based on chemical dosage and pump delivery (pump capacity x chemical concentration) as follows:

Pump Speed (%) = [Effluent Flow (mgd) x Dosage (mg/L) x C]/Pump Delivery (lbs/day)

Where:
C = Units Constant of 34.78 to calculate pump speed in %
Dosage = Desired polymer dose (typically 1.0 mg/L)
Pump Delivery = Maximum Pump Capacity (78 gal/hr) x Chemical Concentration (0.025 lbs/gal)
Sodium Hypochlorite System6
The sodium hypochlorite system consists one storage tank and three feed pumps. Also included in the pumping system are calibration columns, pulsation dampeners, pressure relief valves, and diaphragm valves. 

The tank is rated for a capacity of 1000 gallons. The tank is filled by chemical delivery trucks which connect to fill connection located at the tank. Sodium hypochlorite flows by gravity to three feed pumps. The sodium hypochlorite is then pumped from the feed pumps to the appropriate dosage location.
Alum Tank6
 
The alum system consists of a 5,000 gallon XLPE storage tank and a motor-driven, air-diaphragm chemical feed pump.
Polymer Tank6
 
The polymer system consists of a 2,500 gallon XLPE storage tank, gear drive tank mixer mounted on a mixer bridge, dry polymer eductor, dilution/mixing water system, and a progressing cavity polymer feed pump.
Chemical Feed Pumps 6
Peristaltic metering pumps move magnesium hydroxide and alum from their adjacent storage tanks to the appropriate application point.

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Last Updated: 9/16/2013 9:22:26 AM
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