Wastewater Treatment Primary and Secondary Treatment Primary Settling

Wastewater Treatment: Primary and Secondary Treatment

Primary Settling Basins

Primary Settling

Primary Settling Tank Design • Size – rectangular: 3 -24 m wide x 15 -100 m long – circular: 3 -90 m diameter • Detention time: 1. 5 -2. 5 hours • Overflow rate: 25 -60 m 3/m 2·day • Typical removal efficiencies – solids: 50 -60% – BOD 5: 30 -35%

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 + O 2 → CO 2 • Objective is to allow the BOD to be exerted in the treatment plant rather than in the stream

Diverse Microbial Community • 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, p. H, nutrients (design and operation) • No toxic chemicals present (control industrial inputs)

Dispersed growth vs Fixed Growth • Dispersed Growth – suspended organisms – Activated sludge – Oxidation ditches/ponds – Aerated lagoons, stabilization ponds • Fixed Growth – attached organisms – Trickling filters – Rotating Biological Contactors (RBCs)

Activated Sludge • Process in which a mixture of wastewater and microorganisms (biological sludge) is agitated and aerated • Leads to oxidation of dissolved organics • After oxidation, separate sludge from wastewater • Induce microbial growth – Need food, oxygen – Want Mixed Liquor Suspended Solids (MLSS) of 3, 000 to 6, 000 mg/L

Activated Sludge w/w Return Activated Sludge (RAS) Waste Activated Sludge (WAS) Mixed Liquor Air Treated w/w Secondary clarifier Discharge to River or Land Application

Activated sludge East Lansing WTP

Secondary Clarifier East Lansing WWTP

Activated Sludge Design • Major design parameter: food to microorganism ratio:

Activated Sludge Design • td = approximately 6 - 8 hr • Long rectangular aeration basins • Air is injected near bottom of aeration tanks through system of diffusers • Aeration system used to provide mixing • MLVSS and F/M controlled by wasting a portion of microorganisms

F/M Parameter • Low F/M (low rate of wasting) – – – Starved (hungry) organisms more complete degradation larger, more costly aeration tanks more O 2 required higher power costs (to supply O 2) less sludge to handle • High F/M (high rate of wasting) – organisms are saturated with food – low treatment efficiency

Trickling Filters • Rotating distribution arm sprays primary effluent over circular bed of rock or other coarse media • Air circulates in pores between rocks • “Biofilm” develops on rocks and microorganisms degrade waste materials as they flow past • Organisms slough off in clumps when film gets too thick

Trickling Filters Filter Material

Trickling Filters • Not a true filtering or sieving process • Material only provides surface on which bacteria to grow • Can use plastic media – lighter - can get deeper beds (up to 12 m) – reduced space requirement – larger surface area for growth – greater void ratios (better air flow) – less prone to plugging by accumulating slime

Trickling Filter Plant Layout

Rotating Biological Contactors • Called RBCs • Consists of series of closely spaced discs mounted on a horizontal shaft and rotated while ~40% of each disc is submerged in wastewater • Discs: light-weight plastic • Slime is 1 -3 mm in thickness on disc

Rotating Biological Contactors

Rotating Biological Contactors Aeration Film mixes with wastewater Shearing of excess microorganisms Attached microorganisms pick up organics

Rotating Biological Contactors Primary Settling Sludge Treatment Secondary Settling Sludge Treatment

Low-tech solutions • Aerobic ponds • Facultative ponds • Anaerobic ponds

Aerobic ponds • • Shallow ponds (<1 m deep) Light penetrates to bottom Active algal photosynthesis Organic matter converted to CO 2, NO 3 -, HSO 4 -, HPO 2 -, etc. 4

Facultative ponds • Ponds 1 - 2. 5 m deep • td = 30 - 180 d • not easily subject to upsets due to fluctuations in Q, loading • low capital, O&M costs Aerobic Facultative Anaerobic

Oxidation Ditches

Anaerobic Ponds • Primarily used as a pretreatment process for high strength, high temperature wastes • Can handle much high loadings • 2 stage: – Acid fermentation: Organics Org. acids – Methane fermentation Org. Acids CH 4 and CO 2

Land Wetland Application • Spray irrigation and infiltration • Overland flow • Wetlands Source: Environmental Science, 4 th ed. , B. J. Nebel and R. T. Wright, Prentice. Hall, N. J. , c. 1981

Spray irrigation Secondary Treatment Flooding, channeling spray irrigation • Usually follows oxidation ponds, aerated lagoons • Application leads to filtering, biological degradation, ion exchange, sorption, photodegradation • Need about 1 acre/100 people

Spray irrigation • Problems – climate – pathogens – need buffer zone Source: Environmental Science, 4 th ed. , B. J. Nebel and R. T. Wright, Prentice-Hall, N. J. , c. 1981

Overland flow Secondary Treatment Application to land slopped at 2 -8% • Water irrigated onto long narrow fields • Use grasses that take up large amounts of nitrogen • Underlying soil should be fairly impervious

Overland Flow: • Treats 1 MGD on 200 acres • Settling pond then irrigated • Fields planted with reed canary grass • Below ~1 ft topsoil is compacted clay Source: Environmental Science, 4 th ed. , B. J. Nebel and R. T. Wright, Prentice-Hall, N. J. , c. 1981

Overland Flow: • W/W applied to one side of field, percolates through topsoil to a collecting gutter • Water in gutter (clear and nutrient-free) • Collected in another reservoir and spray-irrigated onto forage crops Source: Environmental Science, 4 th ed. , B. J. Nebel and R. T. Wright, Prentice-Hall, N. J. , c. 1981

Overland Flow: • Advantages – free water – free nutrients – plants can be fed to animals – low-cost – low-maintenance – water meets discharge regulations • Disadvantages – will not work in cold climates – pathogen dispersion in air – need buffer zones – need large amount of land

Wetlands • Use of natural or artificial wetlands • Floating plants act as filters and support for bacteria (From: Environmental Science, 4 th ed. , B. J. Nebeland R. T. Wright, Prentice-Hall, N. J. , © 1981)

Facility Options • Considerations for wastewater treatment facility options – costs • capital • operation and maintenance (including energy) – availability of space – degree of treatment required – municipal or municipal plus industrial – Flow rate

Facility Options • Considerations for wastewater treatment facility options – distance from residential properties • problems with: odors, flies, other nuisances – agricultural usage or land application options – presence of pathogens – experience of design engineers