imn 05 lowcost wwtp

7/21/2019 IMN 05 Lowcost WWTP

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Low cost treatment systems for

wastewater

Indumathi M Nambi

Options Available

• Septic Tanks

• Waste Stabilisation Ponds

• Oxidation ponds

• Aerated lagoons

• Anaerobic ponds

• Facultative ponds

Facultative lagoons

• Constructed Wetlands

– Vertical Flow

Horizontal Flow

Septic tank

Manhole

Definition of “Septic System”

• A subsurface sewage disposal system which usesa combination of a septic tank and a effluentdispersal mechanism.

A two-chamber septic tank is used to accumulatesolid matter. The solid matter is decomposed byanaerobic bacteria. Clear effluent then passes tothe dispersal mechanism, which may beleachlines or seepage pits.

• Septic tanks need to be pumped at least onceevery four years to remove solids.

Soil Permeability

Soil percolation tests

• Soil must be able to accept water over

time in order for a septic system to work

properly.

• Clay soils have poor percolation rates,

and may not support a septic system.

Types of dispersal mechanisms• Leachlines – trenches 3’– 5’ deep with 1-2”

rock under and around 4” perforated pipe.

• Vertical Seepage Pits – 4’ diameter rock-filled pits. The bottom of the pit is kept at

least 10’ above groundwater. Vertical pitsare only allowed in areas with poor qualitygroundwater (basically along the coastlinewhere salt water intrusion has occurred) .

•Horizontal Seepage Pits – A series of 5’diameter rings that are 6’ tall. Soil must bevery permeable (perc rate < 30 min perinch).

Groundwater Protection

requirements• 5’ separation required from leachlines to groundwater.

• 10’ separation required from Horizontal or VerticalSeepage Pits to groundwater.

• 100’ setback required from any portion of a septicsystem to a water well.

• 100’ setback required from a septic system to a year-around stream

• 50’ setback required from a septic system to aseasonal stream.

Surface discharge requirements•

All sewage effluent is to remain underground. If aseptic system fails, the septic tank should bepumped as necessary to keep sewageunderground until repairs can be made.

• A repair to a failing system usually consists of a

200’-300’ leachline addition to the existingleachfield.

• County Code requires a failing system to berepaired within 30 days.

Prefabricated Septic Tank

Tank tied to leachfield

Example of a leachline

What can go into a septic tank?

• Domestic waste may go into a septic tank• Industrial waste, solvents, pesticides or

fertilizers should not go into a septic tank.

• The introduction of toxic materials into aseptic tank will kill the biomat layer in the

tank. The biomat layer then breaks up and

goes into the leachline trenches, cloggingthe system up. This leads to premature

failure.

The minimum hydraulic detention timeshall be two days (48 hours)

In no case shall the septic tank effective

liquid capacity be less than 1000 gallons

Septic Tank Sizing

Example:

School without cafeteria or gym showers whichhas 150 students.

Table 1 = Schools = 10 Gal/person/day

# Students = X 150 persons

Total flow = 1,500 gal/day

Two days retention = X 2 days

3,000 gallon septic tank

Example #2:

Calculate a Restaurant with 20 seats.

Table 1:

Restaurant (per seat)= 50 gal/per/day

X 20 seatsTotal flow = 1000 gal/per/day

Two days retention = X 2 days

Size tank = 2,000 gallon tank

Minimum requirements for

Grease Traps

Two (2) days (48 hr.) retention

MINIMUM CAPACITY 1,000 gal.

(Same as regular septic tanks)

Sizing the Grease Trap

Size of grease trap for 800 gal/day flow of kitchen

waste from a restaurant:

- 800 gpd x 2 days = 1,600 gal. grease trap required.

Size of grease trap for 75 gal/day flow of kitchen

waste:

- 75 gpd x 2 days = 150 gal. therefore GT must besized at 1,000 gal. to meet minimum.

Alternative Systems

• An “alternative” septic system consists of an aerobic

septic tank and a dispersal mechanism. (conventional

systems use anaerobic septic tanks).

•Oxygen is added to the aerobic tank by variousmethods. The down-side to aerobic systems is that

they have moving parts and require electricity. This

opens the door to break-downs and/ or human error.

• The positive aspect is that effluent quality isexponentially better.

Alternative Systems (continued)

• Since effluent quality is better, it may be possible to

reduce the size of the dispersal field. Sizing of

alternative systems is in its inceptive stage, and is still

being worked out..• 4’ separation to groundwater will be required.

Single Oxidation Pond

Function Of Facultative Pond

Photograph of Oxidation Pond

Ponds in series

Design

Completely mixed Reactor with no solids recycle

Sn/So = (1+ k T/n)-n

Sn = Final BODSo = Initial BOD

K= Reaction rate coefficent

T = Hydraulic retention time

N = No of ponds

Aerated lagoon:

Oxygen supplied = 2 kg of Oxygen per kg of

BOD removed

What: Decentralized, low-energy, low-cost

systems to improve water quality

How: Rely on natural wetland function -plants and microorganisms uptake & break

down wastewater nutrients, an- & aerobic

Why: Provide multiple benefits - habitat,water quality, recreation, education,

aesthetic/amenity value, water security &

reuse, CO2 reductions

Constructed Wetlands

Constructed wetlands

Constructed wetland

A commercial water-purifying pond,

planted with Iris pseudacorus

Constructed wetland new & after 2 yrs

How Aquatic Plants Remediate

• Reduction-Oxidation in oxygenated

Rhizosphere (toxic trace metals)

• Accumulation of excess nutrients (N,P)

into plant tissue

• S, Fe, Cu, Se

Nutrient Uptake by Wetlands from

different climatic regions

Types of constructed wetlands

– SubSurface Flow Systems ~0.6m

• Common in Europe

– Surface Flow Systems ~0.4m

•More common in US/North America

• Marsh-like

– Vertical Flow Systems

• New design used to overcome oxygen depletion problem and

boost nitrification

Combined treatment ponds commercial

Combined treatment ponds - commercial

systems

Surface-Flow Treatment Wetlands

• Natural Flow Treatment Wetlands

– Attempts to recreate a natural wetland

– Water source is controlled.

– More useful on large scale

– Effective when excess nutrients

– Trace metals remain in soil after harvest (root

to stem ratio)

Vertical-Flow Treatment Wetlands

• Plants & Soil

– Separate from Natural Environment

– Can remove Soil and Plants during harvest time(iron lines)

• Contaminated Water

• Lots of Control• Expensive Compared to Surface Flow

Vertical Flow Wetlands

Wetland Design & Hydrology

- Basic understanding of environmental factors,

and their interactions is important for the

design and construction of a wetland.- contaminants

- Soil treatment processes

- Water treatment processes- Plant treatment processes

- The wetland needs to be designedaccording to

- filteration- adsorption

- sedimentation

- chemical process,

- biological processes etc

- In addition design principles need to address

- hydraulic load rate

- residence time

- plant density

- inlet concentration C0

- E.g. One can roughly calculate the area needed for

a domestic sewage using the ff equation

(Vymazal et.al, 1998)A = Q d(lnCo – lnCt) / KBOD

where A = area

Q d= ave flow (m3

/day)Co & Ct = influent & effluent BOD (mg/L)

KBOD = 0.10

Wetlands for River Clean up

Biomass

• What happens to the plants after they absorb

these pollutants?

– Controlled burns

– Decomposition – Harvested then burnt

Habitat Creation

• Though built to treat wastewater,constructed wetlands provide habitat for:

– Birds

–Mammals

– Reptiles and Amphibians

– Crustaceans

– Fish

Potential Risks Involved

• Mosquitoes

– Risk of West Nile virus, malaria, and other mosquito-

transmitted diseases

– Constructed wetlands are by nature prime mosquito

habitat

– Two types

• Stagnant water mosquitoes

• Floodwater mosquitoes

– Constructed wetlands more conducive to stagnant water mosquitoes

Mosquito Control

• Methods: – Steep concrete slopes

– Deep bottoms

– Introduction of larvivorous fish

• Mosquitofish (Gambusia affinis)

– Very easily adaptable

– Can cause other environmental problems by out competing otherfish species

– Non mosquito-conducive plants

– Mosquito-specific bacteria (Bacillus thuringiensis andBacillus sphaericus)

imn 05 lowcost wwtp

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