Chapter 11Chapter 11

Wastewater Treatment

On-Site Disposal Systems

Septic Tanks Pit Toilets

Flow Schematic for a Wastewater Treatment Plant

Flow Schematic for a Wastewater Treatment Plant

Preliminary Treatment

Bar screen (bar rack) and/or comminutor Flow measurement Pump Grit chamber

Primary Treatment

sedimentation tank or clarifier Secondary Treatment

Aeration (activated sludge) tank or trickling filter secondary clarifier (sedimentation)

Treatment Levels

Bar Rack

removes large solids such as rags and debris

helps protect pumps, valves, pipelines Communitor

shreds material w/o removing from flow

Bar Screen

Removes dense grit w/o removing less-dense organic solids

Grit: sand, gravel, other inorganic material, and minimally putresible organics (egg shells, coffee grounds, seeds)

Purpose:

prevent excessive wear; abrasion of equipment prevent deposition of grit in pipes/channels prevent accumulation of grit in tanks

Grit Chamber

Included in many treatment plants

Removes much of the settleable solids (grit is usually removed first in grit chamber)

reduces solids flowing to the remaining units

reduces BOD since the solids removed in the tank are organic and have a BOD

Preliminary Clarifier (Settling Tank)

Liquid wastewater sprayed over bed of rocks or plastic

Microorganisms attached to rocks/plastic accomplish biodegradation of waste in flowstream

Air circulates through rock/plastic bed

Trickling Filter

Microorganisms that accomplish biodegradation of waste in flowstream are suspended in the liquid contents of the tank

Air is supplied to the tank for survival of aerobic microorganisms

Activated Sludge

Microorganisms from activated sludge (or trickling filter) settle, producing a clarified effluent leaving the tank

Settled microorganisms in activated sludge are recycled to activated sludge tank

Some microorganisms are “wasted” from the system (to sludge treatment)

Secondary Clarifier (Final Settling in Text)

Chlorination/dechlorination

UV disinfection

Disinfection