2016.4.2 Water treatment：Reed Bed雅思回忆

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Water Treatment 2: Reed Bed

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A

Nowadays

subsurface

flow

wetlands

are

a

common

alternativein

Europe

for

the

treatment

of

wastewater

in

rural

areas.

Mainly

in

the

last

10

to

12

years

there

has

been

a

significant

growth in the number and size of the systems in use. Compared to common treatment facilities,

wetlands are lower in cost investment, lesser to maintain, and are ideal for densely populated

rural or suburban areas rather than urban areas.

B

The Common Reed has the ability to transfer oxygen from its leaves, down through its stem and

rhizomes,

and

out

via

its

root

system.

As

a

result

of

this

action,

a

very

high

population

of

micro-organisms

occurs

in

the

root

system,

with

zones

of

aerobic,

anoxic,

and

anaerobic

conditions. Therefore with the waste water moving very slowly and carefully through the mass of

Reed roots, this liquid can be successfully treated.

C

A straightforward definition of a reed bed is if you have dirty water in your pool or water, which is

heavily

polluted,

Reed

Beds

will

be

planted

to

make

the

water

clean

again.

This

is

good

for

ecologyand living organisms and fish in the water. Reed Beds have a wide range of qualities and

are

acceptable

for

cleaning

everything

from

secondary

to

tertiary

treatment

of

mild

domestic

effluent,

to

rural

waste

and

even

heavy

industrial

contaminants.

The

reason

why

they’re

so

effective is often because within the bed’s root sector, natural biological, physical and chemical

processes interact with one another to degrade or remove a good range of pollutants. Reed beds

can be built in a number of variants, but mainly they are of the horizontal flow or vertical (down)

flow configuration where water flows through the beds horizontally or vertically.

HORIZONTAL FLOW REED BED SYSTEMS

D

Horizontal- flow

wetlands

may

be

of

two

types:

free-water

surface-flow

(FWF)or

sub-surface

water-flow

(SSF).

In

theformer

the

effluent

flows

freely

above

the

sand/gravel

bed

in

which

the reeds etc. are planted; in the latter effluentpasses through the sand/gravel bed. In FWF-type

wetlands, effluent is treated by plant stems, leaves and rhizomes. Such FWF wetlands are densely

planted

and

typically

have

water-depths

of

less

than

0.4m.

However,

dense

planting

can

limit

oxygen diffusion into the water. These systems work particularly well for low strength effluents or

effluents

that

have

undergone

some

form

of

pretreatment

and

play

an

invaluable

role

in

tertiary treatment and the polishing of effluents. The horizontal reed flowsystem uses a long reed

bed, where the liquid slowly flows horizontally through. The length of the reed bed is about 100

meters. The downside of the horizontal reed beds is that they use up lots of land space and they

do take quite a long time to produce clean water.

VERTICAL FLOW REED BED SYSTEMS

E

A vertical flow reed bed is a sealed, gravel filled trench with reeds growing in it (see the picture

below). The common reed oxygenates the water, which helps to create the right environment for