(SEWAGE, EFFLUENT TREATMENT) Pr H.A.Foster October 2013 WASTEWATER TREATMENT
Sources and Sinks of Oxygen in Streams • Tributary flow • Photosynthesis • Plants • Algae • cyanobacteria • Reaeration (weirs) • Oxidation • Carbonaceous Material • Ammonia Nitrogen • Hydrogen sulphide • Benthic Layer (Bottom) • Respiration Sources Sinks Based on a presentation byLeonard W. Casson, Ph.D., P.E., DEE
The Beginnings of wastewater (sewage) treatment (But remember water from washing, body and clothes, preparing vegetables, industrial use, rainwater runoff etc.)
Composition of Wastewaters • Wastewater of domestic origin is usually >99% water with up to 1% solids, both suspended and in solution. • Industrial wastewater streams vary greatly depending on the industry. Abattoirs and food processing plants can produce as much BOD as a small town. • Industrial effluent can also contain toxins.
Preliminary treatment • Removal of dead dogs, foetuses, plastic bottles and other things you cannot pump • Screening through bar screens or perforated plates. • Strainings may be passed through a comminuter (mincer). • Grit Removal – flow slowed to allow grit to settle. • In times of high water flow excess wastewater will pass over weirs and be stored in storm tanks. • Pumping takes energy, sites often sloping to utilise gravity.
Primary settlement = removal of suspended organic matter through sedimentation Flow of effluent slowed in circular or rectangular sedimentation tanks Settled material = sludge. This is removed periodically together with any surface scum Liquid is now termed primary effluent Primary Treatment
Primary treatment -settlement Settlement may be enhanced by addition of polyelectrolytes.
Secondary Treatment of Primary Effluent Biological Processes • Activated Sludge • Deep Shaft • Trickling filter (Biological filter) • RBC (Biodisc) • Aerated Lagoons • Integrated Ponds • Secondary Sedimentation Sedimentation
SE PE Mixed Liquor Measured Here RAS WAS Conventional Activated Sludge Process
Diffuse aeration Mechanical aeration
Primary Secondary Treatment Pretreatment Raw Wastewater Secondary Sed. Biological Treatment Primary Sed. Pre Treatment Disinfection SE PE Effluent Disposal/Reclamation RAS WAS Biosolids Treatment Eluent Treatment Solids Disposal/Reclamation PE = Primary Effluent SE = Secondary Effluent RAS = Return Activated Sludge WAS = Waste Activated Sludge Conventional Wastewater Treatment Unit Processes
Activated Sludge • Variations • Completely mixed (often required nitrifying filter beds) • Extended aeration (>15 h) • Plug-flow (typically 7-8 h residence time) • Organisms grow as ‘flocs’ – macroscopic aggregates of organisms. • Good floc structure is essential for later settlement. Overgrowth of filamentous fungi can cause ‘bulking’ • Production of surfactants can cause foaming or moussing (? Nocardia spp.)
Zooglea ramigera Settled flocs Activated sludge
End Products CO2, H2O Energy for Cell Maintenance Note: Energy is available from The conversion of organic Carbon to CO2 Endogenous Respiration Organic Waste Energy for Cell Synthesis New Cells Nonbiodegradable Residue Yield: Greatest for Aerobic Systems; Less for Anaerobic Systems Aerobic Biological Process Description Leonard W. Casson, Ph.D., P.E., DEE
Expressed in terms of pounds of BOD used per day for each pound of Mean Liquor Suspended Solids (MLSS) in the aeration tank. MLVSS (mean liquor volatile suspended solids) is also used. F/M does not consider BOD in the Return Sludge Sludge age (measured in days) is also controlled Food to Microorganism Ratio (F/M) .
ADVANTAGES OF DEEP SHAFT • Mechanical simplicity. • Low capital and operating costs. • Low land area requirements. • Low environmental impact (low odour, mist and noise). Also largely underground. • High energy efficiency of 2-4kg BOD/kWh.
Deep shaft advantages • Primary treatment not required. • High oxygen transfer rate (up to 3 Kg/m3/hr compared to 0.1 to 0.3 for conventional processes). • High efficiency of oxygen utilisation. • High BOD removal rates. • Process is unaffected by climatic changes.
Deep shaft advantages • Can operate at higher MLSS concentration (3-10 g/l compared to 2-5 g/l) for industrial effluents. • Design sludge loading (kg BOD per day/kg of MLSS) is higher, (0.7 to 1.8 compared with 0.1 to 0.2), and this reduces reactor size. • Limited growth of filamentous organisms means improved sludge settling and smaller clarifiers
Deep shaft advantages • Less sludge mass/volume produced per Kg BOD removed. • No moving parts with low maintenance costs. • Overall cost effective high performance.
Fixed Film Processes Grazing fauna
Role of Microorganisms in the Removal of Organic Matter • Adsorption • Absorption • Respiration (oxidation of substrates for energy) • Synthesis (leading to an increase in biomass)
Waste products CO2, NH3, H2O Small molecules Adsorbed particle Oxygen Enzymes Absorption Energy Respiration Synthesis Structural molecules and enzymes Adsorption to extracellular matrix (capsule and slime)
Organisms found in Activated Sludge • Bacteria • Zooglea • Pseudomonas, Nitrosomonas, Nitrobacter • Beggiatoa • Achromobacter, Flavobacterium, Arthrobacter • Mycobacterium, Nocardia, Herpetosiphon • Escherichia • Leucothrix, Azotobacter, Bacillus • (After Hawkes, 1975)
Protozoa • Important in maintaining flocs and clear effluent • 11 genera of phytoflagellates • 7 genera of zooflagellates • 13 genera of amoebae • 4 genera of actinopods • 59 genera of ciliates.