Waste water Treatment Flashcards
List all the stages in basic water treatment
Raw sewage -> Bar rack -> Grit chambers -> Equalisation Basin (Pre-treatment) ;
- > Primary settling (Primary Treatment)
- > Biological treatment -> Secondary settling (Secondary Treatment)
- > Advanced Treatment (Tertiary Treatment)
- > Receiving Body
What is dry weather flow?
DWF is the average daily sewage flow entering a wastewater treatment plant after 7 days without rain ( less than 25mm on any of the 7 days)
How do you calculate dry weather flow?
DWF = P * G + I + E P-population G-average daily wastewater production I-sewer infiltration E-industrial and commercial discharge to sewer
Define sewer inflow and infiltration
inflow - waters that enters the sewer system through leaky joints, cracks and breaks.
infiltration - storm water that enters the sewer system from storm drain leaders or manhole covers.
How would you calculate sewage flowrate using ‘Formula A’?
Q = DWF + 2*E + 1360*P (Combined sewer) Q = 3*DWF (significant parts of sewer are separate)
How do you calculate the maximum flowrate into an equalisation basin for flow receiving full treatment?
Q = 3PG + 3*E +E
flowrate exceedance is stored in storm tanks then returned to the inlet for treatment after the storm has passed.
What are the two outputs of a wastewater treatment plant?
- Effluent, discharged into rivers or sprayed onto hills.
- Sludge, used as fertiliser or dewatered and treated or used to produce biogas.
What are the criteria for location of a plant?
- close to a river, for easy effluent discharge.
- low elevation, exploiting gravity and no pumping.
- Away from houses, minimize odour issues.
- serve for a large area, reduce pumping and optimize energy use.
Why are Bar racks used in preliminary treatment?
removal of all large objects. solid materials stored in a hopper and removed to sanitary land fields.
Why are grit chambers used in preliminary treatment?
grit (sand, glass, gravel) can damage pumps and others mechanical devices. Consists of long narrow tanks that slow down flow so grit settles out of the water.
Why are equalisation basins used in preliminary treatment?
minimizes the variation in flowrate into the plant so treatment can take place at a constant flowrate, improving the performance and cost.
Why is sedimentation used in primary treatment?
- Grit removal, grit Vs > organic Vsettling (type I settling).
- Primary clarifier, readily settable solids and floating material removed (type II settling)
- Secondary clarifier, sludge generated from biological treatment is removed (type III settling)
Describe the three different settling types
type I: no particle interaction, dilute suspensions.
type II: particles flocculate and settle, velocities increase with time, lab tests necessary.
type III: concentrated particles settle in layers,
name some typical design features for sedimentation tanks
- max Q to full treatment = 3*DWF.
- usually 6 tanks.
- clarifier volume calculated on 6h retention time.
- surface area calculated based on maximum surface loading rate 30m^2/m^2/day at 3*DWF.
- weir length calculated assuming overflow rate of 230m^2/m/day at DWF.
What is the aim of Biological treatment?
remove soluble BOD that escaped primary treatment. secondary settling involves further removal of suspended solids, converting organic matter into microbial mass or inert gases and promoting biomass removal by settling. removal of nutrients
Describe the process of aero decomposition
because of the large amount of energy released in aerobic oxidation, most aerobic organisms are capable of high growth rates. high production of cells is translated into more biological sludge. as the microorganisms grow and mix by the agitation of the air, the organisms clamp together in flocs to form an active mass of microbes called activated sludge.
How do you calculate the biomass concentration at a time t, based on the special growth rate constant?
X = X0 * e^(μt)
μ-special growth rate constant
rg = dx/dt = μX
rg-rate of growth of bacteria
What is the ‘Monod Equation’?
μ = (μm + S) / (Ks + s)
μm-max μ
S-food concentration
Ks-half saturation constant
what is the sludge volume index?
SVI is the volume (ml) occupied by 1g of sludge after 30 minutes of settling. it is the measure of settleability.
SVI (ml/g) = settled volume of sludge/ suspended solids *10^3
SVI = Vs / Xr *1000
Xr = 10^6 / SVI
Xr-recycled sludge concentration
What is sludge bulking and foaming?
occurs if the biological floc has poor settling characteristics (high SVI). usually leads to high suspended solid concentration and poor overall performance in secondary treatment.
what are biofilms?
microbial consortium (bacteria, fungi, algae) that grow on surfaces. they degrade BOD/Cod substrates using oxygen and nutrients. reaction rates are limited by how much organic matter and O2 can diffuse through the biofilm.
What are trickling filters?
TF is an aerobic secondary treatment that uses biofilms attached to a medium to remove organic matter. the organic material is absorbed by the biofilms until it is thick enough that O2 cannot penetrate the medium and anaerobic organisms develop. Eventually parts of the biofilm fall off (sloughing) and these solids are removed.
What are the design criteria for a trickling filter?
single filtration: porous medium (40-60mm), hydraulic loading (500 L/day/m^3), BOD loading (0.12 kg/day/m^3)
Recirculation: porous medium (65mm), hydraulic loading (1200 L/day/m^3), BOD loading (0.18 kg/day/m^3)
How would you calculate the area required for thickening in a trickling filter?
A = Q*tu / Hu
tu-time to reach desired underflow concentration
Hu-sludge thickness when solids at Cu
What are the aims of Tertiary Treatment?
- remove nutrients
- remove poorly degraded chemicals
- disinfection
list the stages of sludge treatment
-> Thickening ->Digestion -> Conditioning -> De-watering
How can you get energy from waste?
Full anaerobic treatment: waste -> CH4 -> electricity (via combustion). <70% efficient, sensitive to low temperatures and biogas quality.
Microbial fuel cells: waste -> electricity (via MFC). coulombic losses <12%. costly
list some anaerobic sludge digestion tank design criteria
- cylindrical tank diameter 6-40m
- water depth 7.5-14m
- suction bottom slope no less 1:12 (v:h)
- gravity withdrawal bottom slope 1:6 (v:h)
