Grundfos fixes off-gassing problem at US plant

Vapour locking at low flow rates was posing problems for Point South wastewater treatment plant, South Carolina. Heidi Berger, sales director - dosing and disinfection for Grundfos, explains how the situation was improved

The Beaufort–Jasper Water & Sewer Authority (BJWSA), is a non-profit utility established by the US state of South Carolina to provide drinking water and wastewater services for the counties of Beaufort and Jasper. The region includes the communities of Hilton Head, Beaufort and Bluffton. Currently BJWSA has two modern water plants and multiple well sites capable of producing up to 148Ml/d of drinking water and six wastewater facilities capable of treating up to 60Ml/d. BJWSA is a leader in water reuse and is capable of reclaiming up to 19Ml/d of water for irrigation, including South Carolina’s first residential irrigation service.

BJWSA operates a lagoon style wastewater treatment plant (WwTP) at Point South, SC.

The 1.3Ml/d plant was upgraded in 2009 to collect and treat wastewater from the small community of Point South, as well as a new planned community designed for 2,500 homes.

Due to the 2009 recession, the planned community was put on hold and to date, no homes have been built. As a result, monthly flow to the new plant averages only 190,000l/d, with night time flows falling to as little as 45l/ minute.

The Point South WwTP plant discharges effluent by pumping to a spray field. BJWSA’s goal is to maintain a 1.5ppm chlorine residual in the final effluent. During the upgrade of the plant, the contractor installed two 0.5HP motordriven mechanical diaphragm pumps, equipped with an auto-degassing liquid end to dose 12.5% sodium hypochlorite.

The mechanical diaphragm pumps, equipped with an internal variable frequency drive (VFD) and stroke length adjustment, were sized for up to 101l/h at 150 psi. The amount of 12.5% hypochlorite required to treat the effluent ranged from a high of 9.5l/h on summer weekends and major rain events to under 0.4l/h during the winter months.

The original pumps worked satisfactorily at higher chemical flow rates, but repeatedly vapour locked at lower flows. To manage the problem of off-gassing and the resultant vapour lock, plant personnel were maintaining higher rates of chemical injection than required, and constantly bleeding off the vapours from the pump head during their morning inspection.

The degassing device on the pumps failed to evacuate the gas from the pump head. Eventually plant personnel were forced to cut the sodium hypochlorite concentration from 12.5% to 6% in order to increase the reliability of the chemical metering pumps.

This manual dilution operation had to be done by the plant operator about every seven days, resulting in increased operational and chemical costs for BJWSA.

The existing system piping was very well done. All accessories, to include a back pressure and pressure relief valve, as well as pulsation dampeners and gages were installed and correctly placed.

There was a vent on the suction line and the suction piping had a gentle upward slope.

Technology
BJWSA was already familiar with the Grundfos DME dosing pump and digital dosing technology. Their engineering group had recently specified two DMEs on an ammonia hydroxide application at a water distribution pump station to “try” the technology. The Purrysburg water treatment plant (WTP) had also purchased a DME to replace an aging hydraulic pump that was feeding fluoride.

When asked about a Beta test opportunity with Grundfos’ new SMART Digital dosing pump, BJWSA mentioned the Point South WwTP and the problems they were having with vapour lock. Their interest was to go back to using full strength 12.5% hypochlorite and avoid the process of diluting the chemical to accommodate the existing pumps.

In this application, the Beta pump (SMART Digital DDA) would be required to follow a 4-20mA signal from an open channel flow meter installed in the chlorine contact chamber. It was agreed that this was the right opportunity to test the new pump’s ability to handle off gassing liquids while operating at
low flows.

Installation
With significant support from BJWSA , the Beta version of the SMART Digital DDA was installed. It was powered up and primed still using the dilute 6% hypochlorite.

The team was pleased to see dozens of hypochlorite bubbles the size of a dime move up the clear plastic tubing through the pump head and out the discharge, an assurance that the pump was performing as desired.

The SMART Digital controls were set to auto Deaeration, Flow Control, and AutoFlowAdapt. These intelligent features allow the pump to automatically adjust the speed to remove air bubbles while matching the capac¬ity set point, even if there are pressure fluctuations in the line. The 4-20mA input was scaled to match the minimum and maximum flow desired.

BJWSA watched the pump closely for a little over a week and noted that it performed perfectly without experiencing a single incidence of vapour lock. The decision was then made to try the SMART Digital DDA on the full strength 12.5% solution.

After the switch to the full strength chemical, the pump continued to perform without a problem. In the weeks following, coastal South Carolina experienced a very hot summer with temperatures often exceeding 37°C and not often below the 33°C range.

Throughout the summer, autumn and winter seasons, the SMART Digital DDA never experienced a vapour lock problem.

Benefits
Beyond the substantial increase in reliability and the ability to meter full strength sodium hypochlorite, BJWSA noted two additional benefits. First, its chemical usage dropped from an average of 41.6l/d with the 6% concentration and down to 7.6l/d with the 12.5% concentration. This has been acknowledged as a cost savings to BJWSA. Secondly, the SMART Digital DDA installation is connected to a universal power supply (UPS), which displays the electrical load. The pump consumes only 1%
of the UPC load.

The original pumps were not reconnected to determine the load, but the plant operator noted that during a power failure the UPC would last only 15 minutes with the motordriven pumps and now lasts 45 minutes with the SMART Digital DDA. By reducing chemical use and power consumption through the use of the SMART digital, BJWSA its environmental impact.

The utility reports that it is extremely pleased with the performance of the Beta version of the Smart Digital.


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