City of Atlanta

Department of Watershed Management

Bureau of Drinking Water

Introduction to

Drinking Water Treatment

The Bureau of Drinking Water oversees all aspects of the water system, including raw water supply, treatment, distribution and water quality compliance.

GLOSSARY OF TERMS

potablemgd (million gallons per day)intaketerbidity - particles and pollutantsNTU - measured unit of turbitity, how dirty the water is based on light reflectivity levelscoagulationflocculation floc - combined weight of dirt and alumsedimentationseasonal turnover - the switch of cool/warm water from top/bottom of reservoir spring/falldistributionsystem control - centralized logistics board, schematic diagram that shows how each stage is functioningfluid mechanicswatershed

History of Atlanta’s Sewer System

The City of Atlanta and the surrounding region have grown tremendously in the last 100 years. While growth is essential to maintaining our economic vitality, it can have negative effects on the environment.

In its earliest days, Atlanta used open watercourses to capture stormwater and divert it into nearby brooks and creeks.

Chattahoochee River, Atlanta’s Water Source

In 1891, the Chattahoochee River was selected as the source for Atlanta’s water by Mayor William Hemphill.

Today the Chattahoochee River is valued more as a source of drinking water and recreation than as a transportation artery.

Facilities – Water Treatment Plants

← Chattahoochee River Intake

Chattahoochee Water

Treatment Plant →

 

Hemphill Water Treatment Plant

Facilities – Water Treatment Plants

Facilities – Water Treatment PlantsChattahoochee River Intake The City currently has a raw

water permit of 180 million gallons per day (mgd) from the GA Dept. of Natural Resources, EPD.

Chattahoochee WTP The CWTP is manually

operated, staffed 24/7 and has a maximum capacity of 64.9 mgd.

Hemphill WTP The HWTP is manually

operated, staffed 24/7, and has a maximum capacity of 136.5 mgd.

Water from the Chattahoochee River Intake enters and flows by gravity to the Chattahoochee Raw Water Pumping Station.

Water is screened and chemicals are applied to it before it leaves the intake.

The water is sent from the intake to the Chattahoochee WTP by the Raw Water Pumping Facility.

The pump station consists of five high-service pumps that send water to the Hemphill Reservoirs and four low-service pumps that send water to the CWTP.

CWTP receives raw water directly from the Chattahoochee Raw Pumping Station.

The current treatment process at the plant consists of flocculation, sedimentation, filtration, disinfection and storage.

Raw water from the Chattahoochee Raw Pumping Station flows into the plant where chemicals are added to promote flocculation.

CWTP has four finished water pumps that send water to the distribution system.

The HWTP is one of three water treatment plants that provides potable water for the City of Atlanta and parts of Fulton County. (The Atlanta-Fulton County Plant also supplies water to Atlanta.)

The HWTP supplies water to retail, residential, commercial, and industrial customers within the City and portions of Fulton County.

Two reservoirs are located at HWTP (No. 1 – cap. 180 mg) and (No. 2 – cap. 345 mg).

As of 1987, seventeen filters have been installed, bringing the filter rate to 136.5 mgd total.

Atlanta’s Raw Water Reservoir at the Hemphill Complex

Water Cycle and Treatment Process

CONVENTIONAL WATER TREATMENT

PROCESS →

Intake Coagulation and Flocculation Sedimentation Filtration Disinfection Storage

From the river to your home, from your home

to the river. ↓

Coagulation and Flocculation

Coagulation and flocculation may be broadly described as chemical and physical processes that mix coagulating chemicals and flocculation aids with water.

The chemistry of coagulation / flocculation consists of three processes:

Flash Mix

Coagulation

Flocculation

Coagulation and Flocculation

Flash MixIn the flash mixer, coagulant chemicals are added to the water and the water is mixed quickly. Flash mixing typically lasts a minute or less.

Coagulation and FlocculationCoagulation

Alum and other chemicals are added to water to form particles called “floc” which attract the dirt particles. The combined weight of the dirt and the alum (floc) become heavy enough to sink to the bottom during sedimentation.

After flash mixing, coagulation occurs. Coagulation removes dirt and other particles suspended in water.

Coagulation and FlocculationFlocculation

During flocculation, a process of gentle mixing brings the fine particles formed by coagulation into contact with each other. This process typically lasts for about thirty to forty-five minutes.

Sedimentation

Suspended particles settle and carry down most of the microorganisms. Clear water is then moved to filtration.

Sedimentation is a physical process used to settle out suspended solids in water under the influence of gravity.

FiltrationThe water passes through filters, some made of layers of sand, gravel, and charcoal that help remove even smaller particles.

Filter Washing

Filter Washing

Post Chemical TreatmentChlorine is added to kill water borne bacteria or microorganisms that are found in untreated water.

Calcium Hydroxide (Lime) is added to increase pH in water.

Phosphoric Acid (Phos) is added to the water to properly coat the pipes for corrosion control.

Hydrofluosilicic Acid (Fluoride) is added to the water to prevent tooth decay.

Clean Water – After Disinfection

Hemphill Water Treatment Plant finished water is stored in three underground clear well tanks. Clear wells #1 and #3 hold 10MG each. The #2 Clear well is our largest storage tank which holds 15MG.

Storage

Pumping to Distribution SystemFinished water is then pumped through various sized pipes to homes and businesses throughout the City of Atlanta and parts of other counties.