The Multi-Barrier Approach to Safe Drinking Water
The key to ensuring clean, safe and reliable drinking water is to understand the drinking water supply from the source all the way to the consumer's tap. This knowledge includes understanding the general characteristics of the water and the land surrounding the water source, as well as mapping all the real and potential threats to the water quality. CWD commissioned a report by the U.S.G.S -Report 00-4262, which details Source-Water Protection published in 2001. This report assesses the factors affecting reservoir and stream water quality in the source area. In addition CWD brought online in 2001 a state-of-the-art treatment plant. CWD has implemented many upgrades to the distribution system to safeguard against operational breakdowns and aging infrastructure. The multi-barrier approach makes sure there are "barriers" in place to either eliminate potential threats or minimize their impact. To safeguard this precious resource CWD has implemented a dynamic Watershed Protection Plan, the use of effective water treatment and proactive management of the distribution system.
The Cambridge WTP employs Pretreatment with Fresh Pond Aeration, Preoxidation, and Dissolved Air Floatation (DAF), Primary Disinfection with Ozone, Filtration using Biologically Active Carbon (BAC) which produces biologically stable water, Disinfection with Chlorine, pH adjustment to produce chemically stable water, the conversion of free chlorine to Monochloramine to provide a stable distribution system disinfectant and preventing water quality deterioration in the distribution system by keeping water age to a minimum.
The approach recognizes that while each individual barrier may be not be able to completely remove or prevent contamination, and therefore protect public health, together the barriers work to provide greater assurance that the water will be safe to drink over the long term.
A brief explanation of each process:
In the pretreatment/clarification step: The addition of Aluminum sulfate as a coagulant, rapid mix, flocculation, and Dissolved Air Flotation (DAF) plant turbidity measurements are used as an indicator of process efficiency. Allied with turbidity removal is the removal of natural organic matter (NOM) like humic acids. NOM contains what is termed Disinfectant Byproduct Precursors. NOM is the raw material that reacts with the disinfectants to produce Disinfectant Byproducts (DPBs) like Bromate, Halomethanes like Trichloromethane and Haloacetic acids like Trichloroacetic acid. DPBs are a health concern and are regulated. The most practical way to quantify these DBP– Precursors effectively is to measure Color, Total Organic Carbon (TOC), and UV254 absorbance, and their removal efficiency.
Turbidity is a process measurement of suspended and colloidal particles including clay, silt, and inorganic matter, algae, and microorganisms. Turbidity is determined by a technique involving the measurement of light scattered at right angles in a water sample. The more of the source light that is scattered the more (the higher) the turbidity. The units of measurement for turbidity are Nephelometric Turbidity Units (NTU’s).
Turbidity is measured through the treatment process as a measure of treatment effectiveness. In the watershed and in the reservoirs turbidity may indicate the presence of silt from storm events or the presence of algae.
The effects of turbidity depend on the nature of the matter that causes the turbidity. High levels of particulate matter may have higher chlorine demand or may protect bacteria from the disinfectant effects of ozone and chlorine, thereby interfering with the disinfectant residual throughout the distribution system. The turbidity through the cycle of each filter run is an indicator of the overall effectiveness of the filter process.
Dissolved Air Floatation
Dissolved Air Flotation (DAF) system creates millions of micro-bubbles to float solid particles to the surface of the water where they are removed as floating sludge, by hydraulic and mechanical skimming. The process starts with the addition of a chemical coagulant and rapidly mixing the raw water. The coagulant Aluminum sulfate (Alum) is introduced, the water is mixed to form Aluminum hydroxide, a precipitate and sulfuric acid which changes the pH of the water from a neutral pH to slightly acidic pH. The Aluminum hydroxide precipitate is mixed in the water to create particle collisions in a process called flocculation. Good coagulation is one of the most important factors affecting flotation. Two-stage tapered flocculation is used to cause the particles to collide and form flow particles using low tip speed to prevent floc from being sheared. The micro-bubbles attach to floc in the water and float it to the surface for removal. The tiny bubbles rise through the coagulated water, capturing floc as they ascend to the surface. The tiny bubbles rise under laminar flow at a rate following a modified Stokes equation. A saturator is used to take a fraction of the throughput and recycle it back to a pressure vessel. A compressor provides air to the tank and mixes with the water to collect in the tank reservoir as saturated air. The aerated water is delivered to a distribution header that spans the width of the DAF cell. The header has a series of specially designed nozzles. The pressure drop through the nozzle produces a cloud of micro-bubbles approximately 20 to 60 microns in diameter. Air under pressure is dissolved into water according to Henry's Law of Dissolution.
Primary Disinfection - Ozone
Intermediate Ozone: In this step ozone is generated onsite and introduced as fine bubbles of ozone that are dissolved into the water and disinfect the water by killing bacteria, viruses, and protozoa. The ozone is introduced into the water in a series of chambers that allow contact and mixing of the ozone with the water. At the end of this process all the ozone introduced is removed from the water.
The CWD provides disinfection to achieve the EPA requirement for 99.9% inactivation of Giardia cysts and 99.99% inactivation of viruses in drinking water. Instead of measuring or counting Giardia and viruses, compliance is determined by a system operational standard, the measurement of the disinfection process. EPA has established a set of criteria for each disinfectant (ozone, free chlorine, and chloramines).
They are stated as CT values where C is concentration and T is time. The concentration C of the disinfectant in the water over time T yields a measure of the effectiveness of disinfection, CT. The required CT varies with the disinfectant type, water temperature, pH, and other factors. CWD measures CT in three places, intermediate ozone, free residual chlorine in the clearwell, and chloramines through the Payson Park Reservoir. The goal is to meet the minimum CT requirements with the intermediate ozone system at a concentration of 1.5 mg/L (milligram per liter) Ozone. The CT credited from the other two sources provides redundancy to the system. The following two graphs show the combined ozone and free chlorine CT.
The presence of natural organic matter (NOM) in drinking water leads to conditions that promote the regrowth of microorganisms in distribution systems. In addition, NOM reacts with disinfectants to form disinfection by-products which present health concerns. Biofiltration can remove much of the biodegradable NOM, which results in decreases in the regrowth potential of water and in significant reductions in the formation of disinfection by-products. CWD has 6 such filters, the filter media is Granular Activated carbon. The filtration typically through all filters is approximately 2.5 gallons per minute per square foot and permitted to do 5 gpm/sq ft.
Disinfection with Chlorine
After the filtration process, the combined filter effluent , water exiting 6 individual filters enters a 400,000 gallon clearwell and chlorine is added. The combination of the concentration of chlorine added and the amount of detention time in this tank the water is disinfected again. A 15% solution of Sodium hypochlorite is added at a concentration of 3.5 mg/L at the entrance the clearwell. The typical chlorine demand is approximately 1 mg/L this leaves a free residual chlorine concentration of 2.5 mg/L available for disinfection in the clearwell.
Secondary Disinfection - Chloramines
Regulations require a minimum of a 0.2 mg/L concentration of disinfectant throughout the distribution system. The higher values reflect the operational need for disinfection with free chlorine after the biological filters. It is this concentration of chlorine that’s mixed with ammonia to create the level of chloramines measured as total residual chlorine using the HACH DPD Method.
The practice of adding ammonia to chlorinated water is called Chloramination. This process is recognized for taste and odor control to reduce the undesirable medicinal taste of chlorinated water. It was first used in Greenville, Tennessee in 1926. This process can contribute to taste and odor control problems if not properly controlled. The formation of di- and tri-chloramines species is minimized by controlling the chlorine and ammonia ratios (3 to 4:1). A 30 % solution of Ammonium Hydroxide is added at a concentration of 0.5 mg/L. CWD’s target chlorine to ammonia ratio is 4.5:1
Corrosion Control – pH adjustment
The National Primary Drinking Water Regulations (NPDWR) - Lead and Copper Rule establishes limits to the amount of lead and copper that may be in drinking water at the consumers tap. The Action Level for Lead is 15 ug/L (micrograms per liter). CWD is in compliance with the 2011 round of reduced sampling. CWD’s 90th percentile is 7 ug/L. The Action Level for Copper is 1300 ug/L. CWD’s 90th percentile was 35ug/L. Cambridge meets the requirements by reducing corrosiveness of the water by adjusting the to pH 9 with a 50% solution of Sodium Hydroxide (as of 1/26/09) at a concentration of 14 mg/L. This combined with the natural occurring alkalinity, hardness and dissolved minerals in the water minimizes the leaching of lead and copper from service lines and home plumbing systems, the source of lead and copper at the consumer tap. The target for distribution system pH is 9.1.
The Massachusetts Department of Health mandates that Drinking Water Systems fluoridate for the prevention of dental cavities. CWD adds a solution of 23% Hydrofluocylicic acid at a concentration of 1.0 mg/L. CWD targets the concentration at 1.1 mg/L.
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