Encyclopedia of Environmental Science and Engineering, Volume I and II

NATURAL SYSTEMS FOR WASTEWATER TREATMENT 739

Anaerobic ponds Anaerobic ponds receive such a heavy

organic loading that there is no aerobic zone. They are

usually 2.5 to 5 m in depth and have detention time of 20 to

50 days. 2,3 The principal biological reactions occurring are

acid formation and methane production. Anaerobic ponds

are usually used for treatment of strong industrial and agri-

cultural wastes, or as a pretreatment step where an industry

is a significant contributor to a municipal system. They do

not have wide application to the treatment of municipal

wastewater.

Facultative ponds It is the most common type of pond

unit. These ponds are usually 1.2 to 2.5 m (4 to 8 ft) in

depth with an aerobic layer overlaying an anaerobic layer,

which often contains sludge deposits. The usual detention

time is 5 to 30 days.^2 Anaerobic fermentation occurs in the

lower layer, and aerobic stabilization occurs in the upper

layer. The key to facultative operation is oxygen produc-

tion by photosynthetic algae and surface reaeration. The

algae cells are necessary for oxygen production, but their

presence in the final effluent represents one of the most

serious performance problems associated with the facultative

ponds.^7

Wetland Treatment Systems

The wetlands are inundated land areas in which water table

is at or above the ground surface (usually 0.6 m or more).

This water table stands long enough time each year to main-

tain saturated soil conditions and also to support the growth

of related vegetation. The vegetation provides surface for

attachment of bacteria films, and aids in the filtration and

adsorption of wastewater constituents. Vegetation also trans-

locate oxygen from leaves to the root systems to support a

wide range of aerobic and facultative bacteria and controls

the growth of algae by restricting the penetration of sunlight.^9

The unique ability of wetland plants to translocate oxygen to

support a wide range of bacteria in the wetlands is shown

in Figure 1. Wetlands can be of two types: natural wetlands

and constructed wetlands. Both natural and constructed wet-

lands have been used for wastewater treatment, although the

use of natural wetlands is generally limited to the polishing

or further treatment of secondary or advanced wastewater

treated effluent.^2

Natural wetlands From a regulatory standpoint, natural

wetlands are usually considered as part of the receiving

waters. Consequently discharges to natural wetlands, in most

cases, must meet applicable regulatory requirements, which

typically stipulate secondary or advanced wastewater treat-

ment.^10 Furthermore, the principal objective when discharg-

ing to natural wetlands should be enhancement of existing

habitat. Modification of existing wetlands to improve treat-

ment capability is often very disruptive to the natural ecosys-

tem and, in general, should not be attempted. 10,11

Constructed wetlands Constructed wetlands offer all of

the treatment capabilities of the natural wetlands but with-

out the constraints associated with discharging to a natural

ecosystem. Additionally, the constructed wetland treatment

units are not restricted to the special requirements on influ-

ent quality. They can ensure much more reliable control

over the hydraulic regime in the system and therefore per-

form more reliably than the natural wetlands.^10 Two types

of constructed wetlands have been developed for wastewater

treatment (1) free water surface (FWS) systems, and (2) sub-

surface flow system (SF). The schematic flow diagrams of

both types of system have been shown in Figure 2. A gen-

eral description of both types of systems is provided below.

The basic design considerations such as (a) site selection,

(b) plants types, (c) physical facilities, (d) hydrologic fac-

tors, (e) organic loading factors, and (f) performance expec-

tations, that are presented under a separate section entitled

Basic Design Considerations of Constructed Wetlands.

Free water surface (FWS) wetlands The free water surface

wetlands typically consist of a basin or channels with some type

of barrier to prevent seepage, soil to support the root systems

TABLE 2

Design features and expected performance for pond treatment units

Parameter

Aerobic

(High Rate)

Aerobic–anaerobic

(Facultative) Anaerobic

Detention time, days 5–20 10–30 20–50

Water depth, m 0.3–1 1–2 2.5–5

BOD 5 loading, kg/ha.d 40–120 15–120 200–500

Soluble BOD 5 removal, percent 90–97 85–95 80–95

Overall BOD 5 removal, percent 40–80 70–90 60–90

Algae concentration, mg/L 100–200 20–80 0–5

Effluent TSS, mg/L 100–250a 40–100a 70–120a

a TSS is high because of algae. Effluent quality can be improved significantly if algae is

removed.

Source: Adopted in part from Refs. 3, 4 and 8.

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