The Treatment Process

Each of HRSD's 13 treatment plants is unique. While the size and layout of the pumps, pipelines and tanks is different, each plant is designed to speed up the process by which water cleanses itself. What happens at a treatment plant is similar to what occurs naturally in a river, lake or stream. This section provides an overview of how our facilities clean wastewater every day for a better Bay.

Wastewater typically is treated in two basic stages known as primary and secondary treatment. In some cases, advanced treatment is required. During primary treatment, solids are screened and settled out of the wastewater. In secondary treatment, bacteria and other small organisms consume the waste and help clean the water. Sometimes, these stages are combined into one operation.

Primary Treatment

When wastewater enters the treatment plant, it flows through a screen that removes large floating objects such as trash, sticks and rags. The wastewater then typically flows to a grit chamber and a sedimentation tank. These devices slow the flow of the water and allow sand, coffee grounds, egg shells, small stones, human and animal waste solids and other small particles to settle to the bottom.

Secondary Treatment

Secondary treatment facilities speed up the processes of nature, allowing microorganisms (bacteria and other organisms) to remove 80-90 percent of the "organic matter" - or human, animal and plant waste. The most commonly used secondary treatment technique in HRSD plants is the activated sludge process.

An activated sludge process speeds up the work of the microorganisms by bringing air and sludge into close contact with the wastewater in an aeration tank. Over several hours, the organic matter is broken down into harmless by-products.

After the activated sludge process, the partially treated wastewater flows to another sedimentation tank to settle out any excess microorganisms.

Tertiary Treatment

Additional wastewater treatment processes are required to remove additional pollutants, such as the nutrients nitrogen and phosphorus. If a treatment facility is to remove 90 percent of the nutrients in the influent (an anticipated requirement for some HRSD facilities), it is necessary to install additional processes that use microorganisms that differ from those in secondary treatment, use additional chemicals and add an effluent filtration system. This significantly increases plant construction and operation costs.

To keep heavy metals, chemical compounds and toxic substances from entering our system, industries that discharge to the sewer are required to pretreat their waste to remove pollutants before they reach HRSD's treatment plants.

Disinfection: Final Step Before Discharge

The final step of treatment is disinfection, usually with chlorine. Chlorination will kill more than 99 percent of harmful bacteria. HRSD facilities remove excess chlorine before discharging the cleaned water.

Laboratory Monitoring Ensures Waterways Are Protected

The final treated wastewater must meet strict state and federal permit requirements established under the Clean Water Act. These requirements are designed to protect both human health and aquatic life in our waterways. Laboratory workers monitor water quality frequently at the wastewater treatment plant, to make sure it meets permit limits.

Solids Handling

Various types of solids are removed as wastewater flows through a treatment plant. The untreated or unstabilized solids are commonly known as sludge. Stabilized solids are called "biosolids." Higher degrees of treatment typically produce larger volumes of solids. Processes that condition, thicken, stabilize and dewater the material are used to facilitate disposal or recycling of the solids. At the end of the process, the concentrated solids can be placed in landfills, incinerated, applied to land or composted for use as a soil conditioner. HRSD uses all of these methods to manage its biosolids.

Sources:
How Wastewater Treatment Works...The Basics, U.S. Environmental Protection Agency, May 1998 (EPA 833-F-98-002)
Clean Water for Today: What is Wastewater Treatment?, Water Environment Federation