Wastewater Treatment• On-site• Municipal

Chemical Oxygen Demand

Oxygen equivalent of the organic matter that can be oxidized by a strong oxidizing agent (potassium dichromate) in an acid medium.

COD > BOD5: (a) Because more compounds can be oxidized chemically than can be oxidized biologically and (b) Because BOD5 does not equal ultimate BODCOD: 3 hBOD: 5 d

On-site wastewater treatment

Septic Systems

Septic Tank – settling, flotation and anaerobic degradation

Septic Systems

Drain field (cross-section) – aerobic degradation

0.04 to 0.05 m3 per capita or 1/3 to 1/2 of septic tank capacity

Municipal Wastewater Treatment Systems

•Pretreatment – removes materials that can cause operational problems, equalization optional•Primary treatment – remove ~60% of SS and ~35% of BOD (by: settle/float) (no removal: soluble pollutants)

•Secondary treatment – remove ~85% of BOD5 and solids(removes: soluble BOD 5, biological processes/speed- up natural processes, not significant N, P, HMs, pathogens)

•Advanced treatment (AWT) – varies: 95+ % of BOD5, SS, N, P, bacteria(Chemical treatment/filtration/land application/soil-crop system)(Sparkling clean, colourless, odourless, effluent)

Bar racks

• Purpose– remove larger objects

• Solid material stored in hopper and sent to landfill

• Mechanically or manually cleaned

Grit Chambers

• Purpose: remove inert dense material, such as sand, broken glass, silt and pebbles

• Avoid abrasion of pumps and other mechanical devices

• Material is called “grit”

MUNICIPAL WASTEWATER TREATMENT PLANT

Primary Clarifiers

Aeration TanksSecondary

Clarifiers

Sludge Thickeners

Return Sludge Pumping Facility

Anaerobic Digestors

Primary SettlingBasins

Primary Settling

Secondary Treatment• Provide BOD removal beyond what is achieved

in primary treatment – removal of soluble BOD– additional removal of suspended solids

• Basic approach is to use aerobic biological degradation:

organic carbon + O2 → CO2

• Objective is to allow the BOD to be exerted in the treatment plant rather than in the stream

How is this accomplished?• Create a very rich

environment for growth of a diverse microbial community

Basic Ingredients• High density of microorganisms (keep

organisms in system)• Good contact between organisms and wastes

(provide mixing)• Provide high levels of oxygen (aeration)• Favorable temperature, pH, nutrients (design

and operation)• No toxic chemicals present (control industrial

inputs)

Wastewater Microbiology1.1 Role of Microorganisms:

Bacteria: single-celled organisms no particular species is selected as “the best”Fungi: multicellular, nonphotosynthetic, heterotrophic require only half as much nitrogen as bacteriaAlgae: uni-/multicellular, net production of oxygenProtozoa: single-celledRotifers and Crustaceans: animals- aerobic, multicellular chemoheterotrophs• Rotifers-consume bacteria, small particles of organics• Crustaceans: shell structure, present in underloaded ponds presence indicate-> DO, << organic matter

ASP/TF

Attached growth/fixed film/stationary microbial populationNo filtering/straining processDepth = 1-3 mDia. < 60 mRocks = 25 – 100 mm dia.Openings too large to strain out solidsSurface area : large where microbes cling/grow/feed on organics

Problems

Under high organic loadings: slime growth – prolific, plug voids, flooding, air circulation/ O2 for microbes – restricted system failureSolution: alternate media modules/ corrugated plastic sheets, plastic rings

Oxidation Ponds

Small communitiesCollective term for all types of pondsOriginally: Oxidation pond – that received partially treated WW Sewage lagoon - that received raw WW Waste stabilization pond – all inclusive pond/lagoon to treat organic WWSelf purification: DO furnished through photosynthesis + surface re-aeration

Classification:

Aerobic – shallow, < 1 m, DO throughout the entire depth, by photosynthesis Facultative – 1- 2.5 m, zones: anaerobic lower, facultative middle, aerobic upper Anaerobic – deep ponds, high organic loadings, anaerobic throughout Maturation/tertiary/polishing –polishing effluents from other biological processes Aerated lagoons – oxygenated through surface/diffused air aeration

Aerobic Ponds.

ShallowLight penetrates to bottomActive algal photosynthesis throughoutDaylight hours – large amount of O2 by photosynthesisDarkness hours–wind mixing of shallow water mass, high degree of surface aerationAerobic bacteria predominate

Anaerobic Ponds.

Aerobic/anaerobic – function of (a) organic loading and (b) availability of DOAnaerobic – BOD5 load > O2 production from photosynthesisTo reduce photosynthesis – decrease surface area, increase depth Turbid – due to reduced metal sulfides,

Reduced light penetration, negligible algal growth

3 stage anaerobic degradation of complex wastes1st stage: hydrolysis of organic matter2nd stage (acid fermentation): complex organics broken to short-chain acids+ alcohols3rd stage (methane fermentation): converted to gases ( primarily CH4 and CO2)

Facultative Ponds:

Most common, small communitiesLong retention – take care of fluctuations in Q and BOD5 Less capital/operating/maintenance costs

Facultative Ponds:

Design

BOD load not > 22 kg/ha.d on smallest cell - To prevent anaerobic conditions DT (considering total volume of all cells, excluding bottom 0.6 m in vol.)=6 months - To provide enough storage to hold WW in winter/when receiving stream is frozen -To provide enough storage to hold WW in summer/when flow in receiving stream is low to absorb even a small amount of BOD.

UPFLOW ANAEROBIC SLUDGE BLANKET REACTOR