Description of Former Water System:

River Crossing is an existing community public water supply consisting of 96 residential apartment units with 1 and 2-bedroom dwellings, with a sewage design flow of 15,840 gallons per day (GPD). Two remotely located wells provide the potable water source for the development. Suction pumps for the wells are located inside a well house almost 1,000 feet away from the apartments. The well house included a potassium hydroxide (KOH) injection system, to raise the low pH, to reduce the corrosivity of the water. Low pH and high corrosivity can cause lead and copper to leach from building plumbing pipes, and River Crossing had experienced elevated lead & copper in the past. 

The KOH treatment system in the well house was old, difficult to control, and had limited chemical safety control equipment. KOH is a highly caustic chemical, and can cause severe burns if an over-feed occurs. Because there was no access drive to the well house, the water operators needed to carry containers of KOH to the well house to maintain the KOH system. This was a challenging and unsafe situation that needed correction. 

Additionally, the water system did not include any atmospheric water storage tank, and the distribution system relied on the two well pumps, plus two additional booster pumps to provide adequate flow and pressure to the apartments. If either well pump or booster pump malfunctioned, the apartments could lose pressure or water service entirely. An underground hydropneumatic storage tank dated back to 1973, which was located inside an underground pit with the two booster pumps, provided some water storage, however, the tank was rusted and could have resulted in a prolonged water outage if it failed. 

Overview of Our Proposed Design Modifications:

We worked with River Crossing to design and implement a comprehensive upgrade of its water treatment, storage, and distribution system by constructing a new booster pump station / treatment building, and an underground 32,700-gallon precast concrete atmospheric storage tank, which will provide more than two days of storage at the 15,840 GPD design flow. We proposed to remove all existing KOH treatment from the well house, to eliminate the need to transport KOH containers to that location, and to install a new bulk KOH tank and new KOH treatment in the new pump station. We also proposed to remove the two existing booster pumps inside the existing booster pump pit, to decommission the hydropneumatic storage tank at that pit, and to re-purpose that pit as a distribution system manifold pit.

Description of New Pump Station & Storage Tank Hydraulics:

In the shoulder of Village Lane, we proposed to intercept the existing dual parallel combined well supply / distribution lines from the well house, and connect them into a proposed 16’x24’ wood-framed pump station building. This new pump station also included a proposed 24’x24’ garage structure for storage of equipment for the grounds operation crew. This new pump station location is in the main part of the site, with excellent access on Village Lane.

The new wells supply line was designed to penetrate up through the floor of the new pump station, pass through a sample tap, a flow meter, and other valves and fittings, before being injected with KOH. A new flow meter was used to pace the KOH injection rate proportionally with the wells flow rate, to maintain a consistent dose rate of KOH. The treated water then passes through a mixing tank, and flows through a 4-inch pipe down through the floor and out to the new 32,000-gallon water tank. 

The storage tank was designed as a precast tunnel tank, with vertical seems, and inside dimensions of 10’wide x 8’tall x 54’ long. The water level depth inside the tank fluctuates normally between 80 and 87 inches high above the floor (i.e. 6.67 and 7.25 feet). A submersible level transducer transmitter was suspended by communication wire from the inside of the tank access riser. The wire was run into the pump station and is used to monitor the storage tank water level.  

Outflow from the storage tank occurs at the opposite end from the inlet, through an outlet suction line through the tank sidewall near the tank bottom. Having the tank supply and outlet on opposite ends of the tank promotes improved turnover and reduces short-cycling and stagnation inside the tank. The suction line connects to two submersible booster pumps, located inside 8-inch wet well casings. The pressurized booster pumps outlet pipes return up through the floor back into the pump station, join together, and include a sample tap, a flow meter, and other valves and fittings. The treated, re-pressurized water then flows down through a distribution pipe through the floor, and connects back to the original two water supply lines in the shoulder of Village Lane. 

Description of Distribution System Connection & Pit Modifications:

The 4-inch distribution line from the pump station reconnects back into the dual parallel 1.5-inch HDPE distribution lines in Village Lane, which run into the existing underground pit near the four existing condominium buildings. As previously described, this pit was being used as a pressure booster pump station, but was re-purposed as a distribution pit, after the existing booster pumps were removed, and the hydropneumatic storage tank was decommissioned by excavating down to it, cutting a hole into it, and injecting flowable concrete to fill the tank volume, before backfilling the excavation. This prevents any potential for the tank caving in due to rusting in the future. 

KOH Injection, Bulk Storage, and Chapter 6 Design:

We designed to inject a 45% solution of KOH (i.e. 450,000 mg/L) at a rate of 0.11 gallons per hour (GPH) at a proportional well flow rate of 27 GPM. We designed the KOH feed pump to be paced on flow as sensed by the combined wells flow meter transmitter, so that the 0.11 GPH dose rate changes when the flow meter changes. We designed a 400-gallon bulk tank KOH storage system so that the site can now accept bulk KOH deliveries less than twice per year, from a licensed hauler, instead of having to continuously lug small jugs around as previous. A KOH transfer pump transfers KOH from the bulk tank to a 30-gallon day tank, by a manual push-and-hold pump activation button. The bulk tank includes a sonic level sensor with a “tank full” audible and visual alarm on the exterior of the building, for the benefit of the bulk hauler, as well as a slightly higher “KOH Overfill Alarm” that calls out as an alarm. This new bulk system essentially eliminates all KOH contact with the water operator.

The KOH day tank also includes a sonic level sensor with a “high day-tank level” alarm, which is electrically interlocked with the KOH transfer pump, so that power to the transfer pump is terminated, and the push-and-hold transfer pump button is disabled when the “high day-tank level” alarm is active. The day tank also includes a “low day-tank level” alarm, which calls out to the operator when activated. The sonic level sensor is also used by the operator to track KOH consumption.

Other safety features include a 12-inch tall concrete containment berm with a containment volume capacity of 536 gallons, which exceeds 110% of the combined 400 gallon KOH bulk tank plus 30 gallon KOH day tank capacity. We have provided a containment area leak sensor and alarm, an emergency eyewash / shower, a pressurized retention tank ahead of the eyewash / shower to allow for stored water to achieve room temperature, and a flow switch alarm that calls out in the event that the eyewash / shower is ever used. We designed an electrical interlock between the “high pH shut-down” alarm and the chemical feed pump and well pumps, so that the KOH feed pump and the well pumps will terminate operation to prevent elevated pH water from entering the storage tank and distribution system, if elevated pH is detected. We also designed a “low pH warning” alarm, and a “high pH warning” alarm, which will not include any interlocks, but call out to the operator to notify of an impending problem.

On-Site Leaching Pit Design:

A leach pit was designed and installed to dispose of the continuous flow-through of water from the pH sensor, as well as to accommodate the discharge of the emergency eyewash / shower and floor drains in the pump station. A test pit investigation was conducted by us to identify bedrock constraints and suitable soils for a leach pit. A suitable location with loamy sand was found, and an infiltration capacity of 3,270 GPD was calculated based on the soil's infiltration rate applied to the bottom area of eight Infiltrator chambers arranged in two rows with 4 chambers in each row.

Controls and Alarm System:

A Programmable Logic Controller (PLC) control panel was used to control essentially all pump station equipment and alarm systems. The main power feed into the pump station originates from an on-site power supply that is already equipped with backup generator, therefore, no new backup generator is anticipated.

The completed System:

The new system was activated in December 2017, and continues to function as designed. The operators are enjoying the improved accessibility and chemical safety equipment. The site enjoys a constant pressure system, with improved reliability and redundancy. 

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