Water Comes from the Powwow River & Enters Our Raw Water Pumps
This area is the beginning stage of treatment. Here, water from the Powwow River flows by gravity to the wet well underneath the Raw Water room. Then, it is pumped by one of two 50 horsepower vertical turbine pumps into the treatment plant to be processed. Well water can also be mixed here to supplement in times of drought.
Rates up to 4.5 million gallons per day (MGD) can be achieved and regulated by variable frequency drives (VFDs). A large stainless steel screen lays on the bottom of the river at the end of the raw water intake pipe. The screen prevents objects and fish from being sucked into the treatment plant; occasionally the screen must be "blown" using an air compressor to dislodge material such as leafs.
Amesbury owns two shallow wells (approximately 50 feet deep). They can supplement the raw water intake with a combined flow of about 1 million gallons per day. Being shallow wells, they are influenced by the nearby surface water source and need to be treated at the "head" of the water treatment plant. This results in a ratio of well and river water, which complicates treatment methods. Also, high iron and manganese levels effect the aesthetic quality of the water Subsequently due to both of these problems, the wells are used only during drought or emergency conditions.
After Entering the Raw Water Pumps, the Water Gets Aerated
This stage of the process introduces air to the raw water to remove volatiles that may be in the water. Also, taste and odor improves at this point. Water is pumped to the top of the aerators and cascades down over hundreds of slats, while air is being pulled up and out.
The Pre-Oxidation (Pre-Ox) tank was part of the 2010 to 2012 upgrade. Built outside the main building, it provides 54,000 gallons or an average of 45 minutes of detention time.
The tank is constructed with baffled walls to provide a "curvy racetrack", which directs the raw water through a long back and forth path. Potassium Permanganate (oxidant) is added at the beginning of the Pre-Ox tank.
In addition, there are 4 submerged turbine mixers, which thoroughly mix the chemicals into the water. All these factors contribute to help remove manganese and iron, which are prevalent in our surface water source.
After the Raw Water Leaves the Aeration Area, It Enters the Flocculation Basin
Here one of the most important treatment processes occurs. Raw water at this point contains:
Organics - from decomposed vegetation
Bacteria and other living organisms
Other varied contaminants
These constitutes must first be removed to become drinking water. Two chemicals are added here to remove "color" (organics); Aluminum Sulfate (Alum) and Sodium Hydroxide. The Alum causes color causing compounds to coagulate (stick to together) and form "floc" (snow flake looking particles). The floc mixers in the chambers slowly mix the floc particles to form bigger floc.
Alum is an acidic chemical which suppresses the pH of the water. Typically in New England, the surface waters have a low alkalinity which will cause the pH to easily fall in the presence of acids. Sodium Hydroxide is a base which is added along with Alum to balance out the pH. At this stage any substances along with organics are formed into floc particles suspended by mixers in the flocculation chamber.
The Next Stage Is the Dissolved Air Flotation (DAF) Area
The Dissolved Air Flotation (DAF) system is one of the more important parts of the production of drinking water for Amesbury water. The upgrade of 2010 to 2012 of the Drinking Water Treatment Facility focused on removing the old sedimentation system and installing the newer DAF technology. Redundancy, more capacity and higher quality water were the result of the upgrade.
Using the below graphic as reference; large floc particles produced in the "2-Stage Flocculation" area enter the diffuser contact chamber, where micro-bubbles from the saturator get attached to the floc particles. The micro-bubbles are created in the saturator vessel from a supply of compressed air and a volume of recycle water are mixed and released through diffuser heads in the contact chamber. The micro-bubbles attach to the floc particles and float up to the surface, forming a "sludge blanket". The clarified water flows to the bottom of the "flotation zone" through plates and out to the filters. Watch a video that shows how water flows through.
Next, the Residuals Are on to the Centrifuge
The centrifuge is a new addition to the Amesbury Water Treatment Facility. Retrofitted in the old 1972 Filter building; the centrifuge processes the Dissolved Air Flotation's (DAF) Centrifuge Conveyor residuals. Over 250 wet tons of sludge are produced each year with the centrifuge and hauled to the fields at Battis Farm on South Hampton Road The sludge is incorporated into one of four fields that are rotated; crops are then grown for forage.
Prior to the use of the centrifuge, Amesbury would have to hire a contractor once a year to process the sludge from lagoons onsite. Now, Amesbury is self-sufficient in its residuals handling.
The operation of the centrifuge is conducted through a sophisticated computer and equipment to produce a dry manageable sludge. Watery residual from the DAF system enters a 40,000 gallon holding tank. Pumps then lift it to the centrifuge where the equipment in conjunction with a polymer, spins the water out of the slurry leaving a dry cake. A conveyor screw moves it to a chute, where it falls into a dump truck. Once a week, the truck hauls it to Battis Farm.
The Water Goes Into Our Filtration System
After water leaves the Dissolved Air Flotation effluent channel, any particles that are lucky to make it out now have to pass through the filters. The filters are designed with five feet of granular activated carbon (GAC). GAC basically looks like black sand. In fact you may have seen it in a fish filter or an inline type filter that attaches to a faucet. GAC provides an excellent media for filtration.The GAC has two properties:
Media for filtration
Adsorption (the process where certain types of molecules will accumulate in "voids" on a particle)
The filter underdrain system water leaving this process is crystal clear. After so many hours the media will become plugged and require a "backwash". The backwash sequence essentially is the action of reversing of clean water through the filter till all the particulates are washed out.
Filter valves - This is the final step where organics are removed; using the special attribute of GAC. After a certain amount of time the GAC will be "spent", because the carbon has adsorbed all the organics it can take. Removal of the GAC then needs to be performed and replaced with new material.
Water exiting the filters enter the 300,000 gallon clearwell, where it used for backwash's and a reservoir for water to be pumped to town. During the upgrade of 2010 to 2012, the underdrain system was replaced with a newer module that enabled replacement of the supporting pea stone gravel with an extra foot of GAC. This improved filtration quality and more organics removal. Newer valves and instruments were also refrofitted.
The Final Stop, the Finished Water Pump Room
This is the last area before the water heads to the city. In the clearwell, which is below the pump room, chlorine is added. A blended phosphate is added after exiting the clearwell. Chlorine is added for disinfection and the blended phosphate is added for corrosion control.
Three high lift pumps are used to convey water to town; one smaller and two larger pumps. The larger pumps are used in times of high demand and fires. During the upgrade of 2008-2012, all the motors and pumps were replaced with energy efficient units.
Next up - learn about the water distribution system!