L5- Stabilisation Ponds Flashcards
Stabilization pond
Artificial large shallow basin to treat WW by physiochemical and biological processes
Advantages and disadvantages of stabilisation ponds and construction issues
Adv: Simple, low cost for equipment & O&M, high removal of pathogens, high algae production
Dis: algae inhibit discharge, large SA required, quality of effluent low, bad odour (anaerobic)
Issues: enbankment, isolation, winds and waves
Overview of Types of ponds
Anaerobic: all water under anaerobic conditions
Aerobic: aerobic conditions maintained for whole volume- oxygen supplied via photosynthesis
Facultative: surface zone aerobic; lower anoxic zone anaerobic; oxygen supplied by photosynthesis
Aerated: Surface aerators supply oxygen and maintain water or part of it in continuous mixing
Pond specifications
Anaerobic: depth 5-10m, production of gas, odour control issues, effluent can’t be discharged, 20-50d residence time
Aerobic: shallow basin (0.3-0.6m)- light penetrate to bottom, exploit photosynthetic power
Facultative: depth 1.5-2.5m, (facultative zone- mixture of aerobic and anaerobic zones)
Aerated: depth 2-6m, treat high organic loads, high efficiency, low growth of algae
Facultative pond zones and bacteria types
Upper aerobic: aerobic bacteria use oxygen from algae and surface aeration
Lower anoxic: anaerobic and aerobic bacteria degrade organics using several oxygen sources
Sludge zone: sludge degradation by anaerobic bacteria
Mechanical aeration system
Mixing zone: maintenance of solids in suspension (inner circle)
Oxygenation zone: diffusion of oxygen in liquid but not mixing (outer circle area)
Considerations: homogeneous distribution, aerators at inlet, no aeration at outlet
Aerobic degradation of organics equation/products
organics+O2+m/o -> new m/o + CO2 + H2O + PO4^3- + NH4+
Anaerobic degradation of organics equation/products
Organics + m/o -> CH4 + H2S + NH4 + CO2
NH4 -> NO2 -> NO3 -> N2 increase
H2S + m/p -> sulphur smells
Operational factors for rate of biological reactions
Sunlight -> O2 and CO2
O2 -> rate
CO2 -> pH -> rate
Removal of pollutants
BOD removal: dissolved (aerobic degradation by bacteria) or particulate (sedimentation, anaerobic degradation)
Suspended solids: mainly in algae, can’t be controlled
Nitrogen: nitrification and denitrification and algae consumption
Phosphorus: low consumption from algae; Al2(SO4)3 added
Temperature effect on BOD removal coefficient & define variables in equation
During summer effect of temperature increased as kinetic rates of biochemical reactions increased
KT- BOD removal coefficient at T (1/d)
K20- BOD removal coefficient at 20deg C
T- water temperature
pheta- temperature coefficient
Complete mix reactor type and formula
CSTR (complete mix = 100%)
S = S0/(1+Kt)^n for qual cells in series
No mix reactor type and formula
Plug flow (PFR)
S = S0*exp(-Kt)
Axial dispersed flow formula
S = S0[(4ae^(1/2d)]/[((1+a)^2)exp(1/2d) - ((1-a)^2)*exp(-a/2d)]
a = sqrt(1+(4Ktd)
Surface area formula
A = V/D = Qt/D
Organic load formula
Lorg = QS0/A = S0D/tow
Volumetric load formula
Lv = BODmass/vol x time = Q*S0/V = S0/tow
Dispersion factor equation & define variables
d = 1/(L/B)
L = length of pond
B = breadth of pond
Dispersion factor for different reactors
d = (D/mu*L)
u = liquid velocity, L = distance between in/out, D= dispersion
d->0 for PFR (no mixing)
d->infinity for CSTR (complete mixing)
Facultative ponds -> d = 0.3-1
Design steps for stabilisation ponds
Given values: S0, k, d, S, Q
Calculate tow
Calculate A = Q*tow/d
Oxygen requirements, OR - define variables
OR- oxygen requirements (kg O2/d)
a- coefficient (0.8-1.2 kg O2/kg BOD)
Q- volumetric flow (m3/d)
S0- total influent BOD concentration (g/m3)
S- soluble effluent BOD concentration (g/m3)
1000- unit conversion
Power requirements, P - define variables
P- power requirements (kW)
OEfield- field operating conditions (0.55-0.65 of OE)
where OE is oxygenation efficiency of aerator at 1.2-2 kg O2/kWh at 20deg C
Removal of pathogens - define variables
N, N0 = concentration of pathogens at effluent and influent (number/100 mL)
KT - microorganisms removal coefficient (1/d)
tow - treatment time (d)
Pathogens inactivation factors
Solar light
High temp and pH
Algae are toxic to m/o
Deficiency of nutrients
Other m/o that consume pathogens
(For Lorg) What is 1 ha converted into m2?
1 ha = 10,000 m2
Reason for series stabilisation pond systems
Optimal efficiency
Reason for parallel stabilisation pond systems
Uniform distribution of flow and solids
Facultative ponds in a row- typical set up
1: initial anaerobic for solids sedimentation
2-4: secondary facultative ponds for BOD removal
5- maturation/disinfection ponds for pathogen removal
