CE3503 Expectations – Wastewater Treatment
Legislative Mandate
- Clean Water Act
o Fishable-swimmable
o Work toward zero discharge
o Technology-based treatment standards
o Total Maximum Daily Loads (TMDLs)
o Water quality-based treatment standards
- National Pollutant Discharge Elimination System
o Issues discharge permits
Wastewater Sources for Publicly-Owned Treatment Works (POTWs)
- wastewater from homes and commercial premises
- infiltration (know definition)
- inflow (know definition)
- problems caused by infiltration and inflow
- industrial sources
o priority pollutants
o problems associated with industrial wastes
§ pass-through pollutants
§ harm to plant
- industrial waste options
o treatment handled by industry
o industry pre-treats and sends to POTW
Wastewater Collection Systems
- connection to house, 6”
- collecting sewers, gather from houses, 8-12”
- interceptor or trunk sewers, to plant, 15-27” or larger
- designed for gravity flow and to –
o prevent in-line sedimentation
o rapid transport, avoid anaerobic conditions
o avoid scour to pipe
Wastewater Characteristics
- BOD and SS, 200 mg/L in, 30 mg/L out
- Ammonia, 25 mg/L in, 2 mg/L out
- Phosphorus, 10 mg/L in, <1 mg/L out
- Coliforms, 108 cells/100 mL in, 100 cells/100 mL out
- Daily variation in wastewater flow
Preliminary Treatment
- know purpose and method for each
o screening (bar racks and screens)
o comminutors
o grit chambers [design problem, Stokes’ Law]
o floatation
o flow equalization
Primary Treatment
- objective and approach
- basis of method
- handling of floatables, sludge
- primary clarifier [design problem, SOR]
Secondary Treatment
- microbiological components
- suspended growth, activated sludge
o aeration tank
o mixed liquor
o mixed liquor suspended solids
o secondary clarifier
o return activated sludge
o waste activated sludge
o F/M ratio [design problem, F/M]
§ growth phases
§ settleability
§ BOD removal
o Sludge age or solids retention time [design problem, SRT]
o Process variations
§ high rate
§ conventional
§ extended aeration
o Process modifications
§ plug flow, tapered aeration
§ plug flow, step aeration
§ completely mixed, conventional
§ completely mixed, contact stabilization
- fixed film
o trickling filters [design problem, TF]
o rotating biological contactors
Tertiary Treatment
- a.k.a. advanced waste treatment or AWT
- suspended solids, filtration
- dissolved BOD, adsorption
- phosphorus, precipitation with alum or ferric chloride
- nitrogen, nitrification to eliminate ammonia
- denitrification, to remove all nitrogen
Disinfection
- chlorination
- ozonation
- UV
Sludge Treatment and Disposal
- stabilization to reduce odor, putrescibility and pathogens
- dewatering to make sludge easier to handle in disposal
- aerobic digestion; long term endogenous phase aeration
- anaerobic digestion
o acid formers
o methane formers
- dewatering
o sand drying beds (small plants)
o belt filter
o centrifuge
- disposal
o landfill
o land application
o re-use (Millorganite)
o incineration
§ fluidized bed
§ multiple hearth furnace
§ co-generation
Summary
Be able to sketch out a schematic of a wastewater treatment plant.
Design Problem 1. Grit Chamber.
A wastewater treatment plant will receive a flow of 35000 m3·d-1 (~10 MGD). Calculate the surface area (m2), volume (m3), and detention time (s) of a 3m deep horizontal flow grit chamber which will remove grit with a specific gravity of >1.9 and a size >0.2 mm.
Calculate the particle settling velocity (vs) using Stokes’ Law.
The design particle (and all larger, denser particles) will be removed if the horizontal velocity within the tank is less than the settling velocity (vh < vs).
Calculate the required surface area of the tank.
Calculate the tank volume.
Calculate the detention time.
Design Problem 2. Primary Clarifier.
A wastewater treatment plant will receive a flow of 35000 m3·d-1 (~10 MGD). Calculate the surface area (m2), diameter (m), volume (m3), and retention time of a 3m deep, circular, primary clarifier which would achieve 50% suspended solids removal. What BOD removal efficiency would be expected?
Calculate the required surface area.
Calculate the tank diameter.
Calculate the tank volume.
Calculate the retention time.
From the design plot, a BOD removal efficiency of ~25% could be expected.
Design Problem 3. Activated Sludge - F/M Ratio.
A wastewater treatment plant will receive a flow of 35000 m3·d-1 (~10 MGD) with a raw wastewater CBOD5 of 250 mg·L-1. Primary treatment removes ~25% of the BOD. Calculate the volume (m3) and approximate retention time (d) of the activated sludge aeration basin required to run the plant as a “high rate” facility (F/M = 2 d-1). The aeration basin MLSS concentration will be maintained at 2000 mgMLSS·L-1.
Calculate the CBOD5 feed to the aeration tank.
Calculate the MLSS concentration of the aeration tank.
Use the definition of F/M and the data above to calculate V.
Calculate the retention time.
This retention time is approximate, because the flow estimate does not include return activated sludge.
Design Problem 4. Activated Sludge - SRT.
A wastewater treatment plant will receive a flow of 35000 m3·d-1 (~10 MGD). Calculate the mass of sludge wasted each day (Qw·Xw, kg·d-1) for an activated sludge system operated at a solids retention time (SRT or c) of 5 days. Assume an aeration tank volume of 1640 m3 and a MLSS concentration of 2000 mg·L-1. What fraction of the solids leaving the aeration tank is this?
Set up the SRT design equation.
Rearrange and solve for Qw·Xw.
Calculate the % of the solids leaving the aeration tank.
Design Problem 5. Trickling Filter.
A wastewater treatment plant will receive a flow of 35000 m3·d-1 (~10 MGD) with a raw wastewater CBOD5 of 250 mg·L-1. Primary treatment removes ~25% of the BOD. Calculate the diameter (m) of the 3m deep trickling filter(s) which would accommodate a hydraulic loading of 5 m3·m-2·d-1 and an organic loading of 250 gCBOD·m-2·d-1.
Calculate the CBOD5 load to the filter.
Calculate the required filter area based on CBOD loading.
Calculate the diameter.
Thus, ~9 filters with an area of 2831 m2 (60m diameter) would be required.
Calculate the required filter area based on the hydraulic loading.
Calculate the diameter.
Thus, ~3 filters with an area of 2831 m2 (60m diameter) would be required.
The design must be made to accommodate both the BOD and hydraulic loads, therefore the BOD design criterion governs here.