PD-Sulfite Dechlorinating - HCMA

In March of 2015, ASTI Environmental was retained by the Huron-Clinton Metropolitan Authority (HCMA) to identify a treatment system to remove chlorine form pool filter backwash water in order to discharge to a Lake Erie Wetland.  Concentrations of chlorine in the pool filter backwash water range from 3.0 mg/l to above 7.0 mg/l.  MDEQ rules limit continuous discharge to 0.038 mg/l. The objective of this project was to find a treatment method that would require minimal maintenance, have a low environmental impact and be cost effective.
ASTI developed cost estimates, construction and operation requirements for different dechlorination technologies including: ultraviolet light; sulphite dechlorination; ascorbic acid treatment; and natural dechlorination. can significantly reduce oxygen levels in the receiving waters if an excess concentration is added to the discharge.  

Sulfite Dechlorinating
Four types of sulfite dechlorinating agents are available:
•    sulfur dioxide (SO2)
•    sodium bisulfite (NaHSO3)
•    sodium sulfite (Na2SO3)
•    sodium thiosulfate (Na2S2O3)
Sulfur dioxide is a gas, and as such is not routinely used, or recommended because of its hazardous nature, for field dechlorination purposes. Sodium bisulfite, sodium sulfite, and sodium thiosulfate exert can significantly reduce oxygen levels in the receiving waters if an excess concentration is added to the discharge.  Automating a sulfite treatment system would require capital expenditures for the mix tank, piping, pump and operational costs for labor and chemical.  The mix tank would require monthly maintenance to add sulfite chemical to the mix tank.

Ascorbic Acid Dechlorination
Ascorbic acid also known as Vitamin C, L-ascorbic acid, Ascorbate or Ascoltin can be used to dechlorinate water and does not have the potential to reduce oxygen levels in the receiving water.  An injection feed tank (approximately 20 gallons), pump and metering system are required and would be operated when the backwash pumps are operating.  The treatment system would require capital expenditures for the mix tank, piping, pump and operational costs for labor and chemical.  The mix tank would require monthly maintenance to add chemical to the mix tank.

Natural Dechlorination – Detention Pond
Chlorinated water will dechlorinate if exposed to the atmosphere and sunlight or if aerated.   Dechlorination rates depend on initial concentration, area of water surface, temperature, time exposed to sunlight and aeration.  In order to minimize maintenance an aeration system was not considered.  There would be significant costs for construction of a detention pond; however operational costs would be limited to collected samples at the discharge point for chlorine concentration analysis.

ASTI’s Process
ASTI worked with key HCMA staff to develop a sample collection plan.  ASTI collected samples of the pool filter backwash, analyzed for suspended solids and dissolved solids because their concentration affects the chemical and ultra violet dechorination process. ASTI also collected samples to determine the effect of exposing backwash water to sunlight and the atmosphere
Samples of pool backwash water were collected and allowed to sit in open containers.  The pH was measured each day at a specified time.  The results indicated that water with chlorine concentration at approximately 3.0 mg/l would naturally dechlorinate to less than 1.0 mg/l in approximately 3 days. 
ASTI concluded that the best dechlorination technique would be to use a detention pond and to expose the pool filter backwash water to the weather and sunlight for a 5-day detention time. ASTI provided the design of the detention pond, including dimensions, location, discharge structure and discharge rate.   
ASTI continues to provide environmental services to the Huron-Clinton Metropolitan Authority.