Chapter 11 Wastewater Treatment

On-Site Disposal Systems Septic Tanks Pit Toilets

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