CE 326 Principles of Environmental Engineering
Wastewater Engineering 2
Primary Clarifier - Sizing
Principle is similar to that of clarifier used in water treatment
Can be circular or rectangular
Type II settling – flocculent settling
Design criteria
Range
overflow rates
25 to 60 m/d
Peak overflow rate
80 – 120 m/d
weir overflow rates
120 m3/d/m of weir
190 m3/d/m of weir
Depth
3–5m
Detention time
1.5 – 3.0 hours
Removes
Typical
100
<0.04 m3/s
> 0.04 m3/s
3.5 m
2 hours
50 – 60% of suspended solids
30 – 35% of BOD5
Secondary Clarifiers- Sizing
Similar to primary clarifiers. However, the secondary clarifiers must be designed to handle a solids load.
Design Criteria
Activated Sludge
Average
Peak
16 – 32
40 – 48
Trickling Filters
Average
16 – 24
Peak
40 - 48
weir overflow rate
(m3/m/day)
125 – 250
--
125 – 250
--
solids loading rate *
(kg/m2/day)
94 – 140
235
70 – 88
188
Depth (m)
3.7 – 6
overflow rate
(m3/m2/day)
3–6
* solids loading rate (kg/m2/day) is given by: (Q + Qr)*X/(1000 * As)
where
Q = influent flow rate (m3/day)
Qr = recycle flow rate (m3/day)
X = concentration of solids in aeration basin (mg/L)
1000 = conversion factor (for L to m3 and mg to kg)
Sludge Treatment
Sources of Sludge at a Wastewater Treatment Plant
Primary or raw sludge
- from primary clarifier
- has offensive odor, slimy and gray
- 2 to 8 % solids
- 60 – 80% of total solids made of volatile solids
- pH = 5 to 8
- density between 1.02 to 1.4
- readily digested
Activated sludge or secondary sludge
- generally consists of microorganisms cells and inert materials
- brownish, flocculent appearance
- readily digested
- Total solids, 0.8% - 1.5%
- volatile solids – 59 – 90% of total solids
- density 1.005 – 1.25
Sludge Management
Wastewater sludge is stabilized to:
- reduce pathogens
- eliminate offensive odors
- inhibit, reduce or eliminate the potential of the organic to putrefy
- reduce the mass of sludge
Technologies available to stabilize the sludge include:
- anaerobic digestion
- aerobic digestion
- lime stabilization
- heat treatment
- composing
Typical Flow diagram for sludge treatment
Thickening
Two types – gravity and by flotation
Gravity thickening
- similar to settling tanks for primary and secondary clarifiers
- purpose to increase the solids content further
unthickened
thickened
influent (%)
Primary Sludge
2 -8
5 – 10
Activated Sludge
0.5 – 1.5
2–3
Primary and
Activated Sludge
0.5 – 4
4–7
solid loading
kg/m2/day
96 – 144
12 – 36
24 – 84
Flotation (equipment is known as dissolved air flotation (DAF))
- separates solids from liquid phase by introducing fine gas bubbles into the liquid phase
- bubbles attached to solids, buoyant forces of the bubble cause the particle to float
- at the surface of the tank, the solids are skimmed off
- for a small system, wastewater is pressurized by pumps to 3 – 4 atmosphere with compressed air
Advantage – very small and light particles that settle slowly can be removed more completely and in a short time
Typical solids loading rate
Primary Sludge (kg/m2/hr)
Activated sludge (kg/m2/hr)
Primary +
Activated Sludge (kg/m2/hr)
with chemicals
295
220
without chemicals
100 – 150
50
220
70 – 150
Anaerobic Digestion
Basic Principles
One pound of BODL will produce 0.25 lbs of methane or 5.62 ft3 of methane
Two types of anaerobic digestion system
Single stage standard rate digester
- small installation (single tank functions
as sludge thickening, separation and digestion)
- results in stratification
- large tank required
- low loading rate
Design information
Solids residence time
30 - 60 days
Loading rate
0.48 – 1.6 kg VSS/m3 of digester volume/day
Two stage high rate digester
- first stage for mixing (temperature will be more uniform)
- second stage with no mixing allows for separation
of digested sludge and supernatant and gas
- tanks can be made to be identical size
Design information
First and second stage – 10 – 15 days each
solids loading
- 1.6 – 2.2 kg/m3/day
Aerobic Digestion
- used for small plants
- needs to aerate sludge for extended period
- similar to activated sludge
- involves direct oxidation of biodegradable fraction of the microbial cellular materials (endogeneous
respiration)
- deisgn criteria
- hydraulic retention time – 15 days to achieve 40 – 50% reduction in volatile solids
Advantages
- simple to operate
- low capital cost
- digested sludge is odorless
- excellent dewatering properties
- supernatant low in BOD
- high operating costs – needs lots of oxygen
Lime stabilization
- add sufficient quantity of lime to raise the pH to >12. Alkaline condition – not conducive for survival of
microorganisms. EPA requirement – maintain pH 12 for 2 hours for pathogen destruction
- sludge will not putrefy, create odors or pose a health hazard
Advanced Treatment (nutrient removal)
Removal of Nitrogen
-Nitrogen in wastewater in the form of organic nitrogen, NH 3, NH4+, NO32-, NO2- Problems
- a nutrient that promotes algae growth in lakes, rivers – resulting in eutrophication of lakes
- ammonia may be toxic to certain aquatic life
- exerts a demand on oxygen (nitrogeneous BOD) – depleting oxygen in streams
- nitrogen is removed from wastewater through nitrification and denitrification.
Nitrification – mediated by bacteria (nitrosomonas and nitrobacter)
NH4+ + 2O2 =
NO32- + H2O + 2H+
Denitrification – conversion of nitrate to nitrogen gas by microorganism
2 NO32- + organic matter = N2 + CO2 + H2O
- nitrification in a wastewater treatment occurs when sufficient solids retention time (SRT) > 10 days is provided.
- in the case of denitrification, separate unit process is needed. Usually at the end of the treatment sequence.
Organic matter is provided by adding synthetic organic such as methanol, or settled sewage.
Removal of phosphorus
- Phosphorus is a nutrient that promotes algae growth, resulting in eutrophication of lakes
- by chemical precipitation such as alum, lime and ferric chloride
HPO42- + Fe Cl3 = FePO4 ↓ + H+ + 3 Cl2HPO42- + Al2(SO4)3 = 2AlPO4 ↓ + 2 H+ + 3 SO42- by biological accumulation, certain microorganisms under anoxic conditions can accumulate phosphorus
in their cells.