Converting a Problem into an Opportunity: Achieving Compliance While Saving Money
Mike Tibbets, PE
Hayter Engineering, Inc.
If we haven't complained about this ourselves, we've heard someone else complain about it:
“My system is so spread out that I can't get a good disinfection residual at the end of my lines without lots of flushing! And that treated water going out on the ground is expensive!”
This complaint is common to many rural water supply utilities across Texas. In many cases, distribution systems have miles of pipe with few customers – resulting in a situation where disinfection residuals are difficult to maintain. And failure to properly maintain these residuals by mandatory flushing can result in fines by TCEQ and highly unpopular boil-water notices being issued to the customers.
Central Bowie County Water Supply Corporation (CBCWSC) found itself in this situation a few year ago. The corporation (which serves approximately 2,800 connections) was required by TCEQ to issue a boil-water notice following an incident in which the minimum disinfectant residual was not achieved. Attempts to resolve this issue, and attempts to not repeat the situation, required extraordinary amounts of flushing just to meet the minimum disinfection residual level.
The water that CBCWSC sends to it customers is purchased from a regional supplier about 30 miles away. Not only was it costly to flush purchased water out on the ground, it was costly in terms of personnel and equipment to travel over 20 miles to each flushing station each day. The Board of Directors wanted a solution that would reduce the flushing and improve the disinfection residual. The Board instructed General Manager, Hal W. Harris, to pursue a solution to the problem.
Harris then reached out to Hayter Engineering, based in Paris, Texas. Hayter Engineering was asked to review the situation, and to propose a solution.
Vice President and Senior Project Manager, Mike Tibbets, met with Harris to evaluate the situation. Tibbets recorded the following observations:
- Source of supply over 30 miles away
- One pipeline served numerous other wholesale water suppliers other than just CBCWSC
- No coordination between wholesale water suppliers as to when water will be drawn from the pipeline
- Extreme variation in the flowrate of water (0 to 600 GPM) delivered to CBCWSC
- No consistency in the chloramine concentration (1.8 to 3.6 mg/L) due to the large number of users on the line
The consequence of the large variation of flowrate and disinfection residual, occurring simultaneously, was that the system operators could not adjust their manual chlorine feed system to match the ever-changing conditions. Not being able to adjust the chlorine feed system meant that legal minimum disinfection level of 0.5 mg/L of total chlorine could not be met on many occasions, and that the maximum level of 4.0 mg/L was also exceeded.
If that was not bad enough, the Corporation was not able to boost the chloramine residual because the only chemical feed they had was chlorine gas (free chlorine). If the Corporation wanted to boost the disinfectant residual, it had to add enough chlorine to “burn out” the chloramines and convert the residual to chlorine only.
To make matters worse, the ground storage tank only had one pipe from which to draw and fill the tank. There was not much turnover or circulation occurring in the tank.
As a result of the conditions described above, CBCWSC experienced great difficulty staying in compliance with TCEQ minimum and maximum disinfection residual levels – so much that boil water notices had to be issued and the Corporation was cited for these violations.
To address the issue of varying flowrate, Hayter Engineering designed a flow-paced chlorine feed system. This design tied the 4-20 mA signal of a new flowmeter to the chlorine feeder so that the chlorine fed into the system was proportional to the volume of water entering the ground tank.
To address the issue of not being able to boost chloramines, a feed pump was designed that fed liquid ammonium sulfate (LAS) into the water before it entered the ground tank. The chemical feed pump was also flow-paced (like the chlorine feed system) so that the amount of ammonia introduced into the system was proportional to the flow rate.
To address the issue of stagnant water in the ground tank due to only one fill/draw pipe, modifications were made to the ground tank piping. The original fill/draw pipe was converted to fill-only. A riser pipe was added to this pipe so that it filled the tank at the top. A second pipe was added on the opposite side of the tank, at the bottom, so that water coming in at the top had to exit at the bottom – thus achieving a much higher degree of water turnover than was the case originally.
An online total chlorine monitor was included in the design. This monitor was tied to the Supervisory Control and Data Acquisition (SCADA) system to provide real time data and alarms. This allowed for remote monitoring of the conditions at the site.
After placing the tank into service, and after several weeks of troubleshooting and operational adjustments, consistent disinfectant residual levels were achieved, as evidenced by the following chlorine residual monitor record:
Note that the residual does not vary widely, as was the case prior to the implementation of the new design. The residual varies from extremes of 4.1 to 3.1 mg/L in this example. The operator stated that a higher and more consistent residual was attained at the far reaches of the system – which in turn reduced the amount of required flushing.
The savings achieved by a reduction in flushing in the first four months of operation is shown below:
|Month||2017 Flushing||2018 FLushing||Savings (Gallons)||Savings||Savings*|
- = raw water cost @ $1.48/1,000 gallons will soon increase to $1.61/1,000 gallons
Compared to the previous year, the average savings realized in the first four months of operation was over $2,700 per month.
If your system is experiencing a similar situation, consider using flow-paced chlorine and chemical feed, perhaps combined with improvements in piping, to improve the consistency of your disinfection residual to reduce the amount of flushing required.
Perhaps your system will find itself as fortunate as Central Bowie County WSC.
Mike Tibbets is the Vice President of Hayter Engineering in Paris. Tibbets has a MS and BS in Civil Engineering from Texas A&M University in College Station. He has over 33 years of experience in the area of water resources engineering as it pertains to municipal and rural governmental entities. He is also the author of Feasibility of Seasonal Multipurpose Reservoir Operation in Texas, among other publications.
Tibbets can be reached at firstname.lastname@example.org or 903-785-0303.
Views in this article may not reflect those of the Texas Rural Water Association or its staff.