Constructed Wetland - Copy

8/12/2019 Constructed Wetland - Copy

1/26

Constructed Wetlands Design

Source: UN-HABITAT

1


2/26

Merits of constructed wetlands

Smaller, decentralized, wastewater management

Relatively inexpensive

Natural system, simple design, construction to build where

land is affordable Easily operated and maintained even by the community

95% rural Europe

2


3/26

Major components of CW

Basina shallow excavation

Substratepermeable filler materialrock, gravel, sand, soil,

supports plant roots

Vegetationsame type of budding plants Linerfor protection of groundwater, if required

Inlet/Outlet arrangement systemsubsurface flow, uniform

distribution and collection of w/w

Pollutant Removal Mechanisms in Constructed Wetlands - By

various physical, chemical and biological processes

3


4/26

Advantages of CW

wetlands can be less expensive to build than other treatment

options

utilization of natural processes,

simple construction (can be constructed with local materials), simple operation and maintenance,

cost effectiveness (low construction and operation costs),

process stability

4


5/26

Limitations of CW

large area requirement

wetland treatment may be economical relative to other

options only where land is available and affordable

design criteria have yet to be developed for different types ofwastewater and climates

5


6/26

Design configurations of CW

Many, based on various factors.

Based on flow pattern in the wetlands:

o free water surface flow

o subsurface flowohorizontal

o vertical

Hybrid systems combine the best advantages of both HF and

VF systems. They may beo HF followed by VF

o VF followed by HF

6


7/26

Horizontal subsurface flow Vertical subsurface flow

w/w flows under the surface of

the bed in a more or lesshorizontal path

w/w is fed from the top and then

gradually percolates downthrough the bed and is collected

at the base

Slow flow through the porous

surface

Intermittent flow in a large batch

flooding the surface

Removal of TSS, BOD, COD,

nitrates

Removal of BOD, COD,

pathogens. Nitrification occurs

Limited oxygen transfer Oxygen transfer is good

Smaller area than HF

More effective than HF

Better removal of TSS

Clogging occurs with incorrect

media

7


8/26

Design of constructed wetlands

Substrate of the wetland can be clogged with debris, grit, and solids

from raw wastewater. Therefore, a primary treatment should be

provided to remove the settleable solids. This may be

o screening and grit removal

o Septic tanks

o Anaerobic baffle reactor (Improved septic tank)

8


9/26

Design of constructed wetlands

Surface area of wetland (Kickuth equation)

= ln ln

Where,

Ah= Surface area of bed (m2)

Qd= average daily flow rate of sewage (m3/d)

Ci= influent BOD5 concentration (mg/l)

Ce= effluent BOD5 concentration (mg/l)

KBOD= rate constant (m/d)

9


10/26

= .

Where,

KT = K20(1.06)(T-20)

K20

= rate constant at 20C (d-1)

T = operational temperature of system (C)

d = depth of water column (m)

recommended values: HF = 40 cm, VF = 70 cm

n = porosity of the substrate medium (percentage expressed as

fraction)

KBODis temperature dependent and the BOD degradation rate

generally increases about 10 % per 0C

10


11/26

Depth

restricted to approximately the rooting depth of plants

HRT (time the wastewater is retained in the wetland) is to be

consideredRecommended depth

HF: ~ 40 cm, considering precipitation, which causes surface

flow

VF: 70 cm, for nitrification

11


12/26

Bed cross section area (HF only)

It is derived from Darcys law - subsurface flow through the gravel

under average flow conditions. Assumptions:

hydraulic gradient = slope, and

hydraulic conductivity will stabilize at 10-3m/s in the established

wetland

Ac= Qs/ (Kf(dH/ds))

Ac= Cross sectional area of the bed (m2)

Qs= average flow (m3/s)

Kf = hydraulic conductivity of the fully developed bed (m/s)

For graded gravel: Kf = 1 x 10-3 to 3 x 10-3 m/s

dH/ds = slope of bottom of the bed (m/m) (usually 1%)

12


13/26

Then, width of wetland = C S Area / depth of wetland

Therefore, length of wetland = total surface area / width of wetland

On partitioning wetland cell into n numbers, the Qeach cell= Qs/ n

Recommendations:

If the width of the wetland is more than 15 m, the wetland cellshould be partitioned to avoid short circuiting of wastewater inside

the wetland.

It is better to use at least two parallel cells instead of a single

wetland cell for the ease in O&M

Note:In VF wetlands, since the flow is vertical, the width and cross-

sectional area of VF beds are not set by a requirement to keep the flow

below surface and prevent surface flow

13


14/26

Substrate selection

Recommended media:

Intermediate-sized materials generally characterized as gravels. The

gravels are washed because to remove fines that could block the void

spaces.

HF wetlands

14


15/26

Substrate selection

VF wetlands

15


16/26

Bed slope

Theoretically, the bottom slope should match the slope of the

water level to maintain a uniform water depth throughout the

bed.

Practical approach: slope the bottom along the direction of

flow from inlet to outlet

slope of 0.5 to 1% is recommended

16


17/26

Sealing of the bed

Coefficient of

permeability k of

native soil

Interpretation

k>10-6m/s the soil is too permeable and the wetlands must

be lined

k>10-7m/s some seepage may occur but not sufficiently toprevent the wetlands from having submerged

condition

k


18/26

Inlet structurescommon types used

HF

Submerged perforated pipe

(such as riser pipes with V-

notches) Gabion feed

Swivel tee

Perforated pipe inlet

Slotted pipe inlet

Channel inlet

VF

Inlet structures for VFwetland comprises of anintermittent feeding tankwith distribution network.

Types of inlets used:

o Hydromechanicalsiphons

o Network of pipes withdownward pointingholes

18


19/26

Outlet structures

The design of subsurface flow wetlands should allow controlled

flooding to 15 cm for desirable plant growth. Types of outlets:

Adjustable weir

Interchangeable section

900Elbow arrangement

Elbow outlets

Flexible pipe outlet

19


20/26

Vegetation

The plants should meet the following criteria

Common in the region

Deep, strong, fibrous roots

Large biomass or stem densities Maximum surface area of roots for supporting microbial

populations

Commonly used: Phragmitessp. and Typhasp., of plants which

includes the popular choicecommon reed

20


21/26

0.075 MLD

Case studiesNepal

The removal efficiencies of 6 CW in Nepal for TSS, BOD,

COD have remained over 80% during 2006 -2007

21


22/26

0.0007 MLD

Case studiesNepal



22


23/26

0.03 MLD

Case studiesNepal



23


24/26

0.01 MLD

Case studiesNepal



24


25/26

0.0005 MLD

Case studiesNepal



25


26/26

0.075 MLD

Case studiesNepal



26

Documents

Constructed Wetland - Copy