Encyclopedia of Environmental Science and Engineering, Volume I and II

738 NATURAL SYSTEMS FOR WASTEWATER TREATMENT

by pumping or under-drain systems for recovery of treated

water, (3) natural treatment with groundwater moving verti-

cally and laterally in the soil, and (4) recharging a surface water

source.^5

Overland flow In the overland flow system, the wastewater

is applied over the upper reaches of the sloped terraces and

allowed to flow overland and is collected at the toe of the slopes.

The collected effluent can be either reused or discharged into

a receiving water. Biochemical oxidation, sedimentation, fil-

tration and chemical adsorption are the primary mechanisms

for removal of the contaminants. Nitrogen removal is achieved

through denitrification. Plant uptake of nitrogen and phospho-

rus are significant if crop harvesting is practiced. 3,5

Aquatic Treatment Systems

Aquatic system utilize lagoons and ponds, and wetlands. The

lagoons and ponds depend on microbial life, and lower plants

and animals, while wetlands support the growth of rooted

plants. Both pond systems and wetlands are discussed below.

Pond Systems

Within the aquatic systems, pond systems are the most widely

accepted ones. Basically pond systems can be of three types

based on oxygen requirement. These are aerobic, anaerobic

and facultative pond systems. In all cases the major treatment

responses are due to the biological components. 2,7 In most of

the pond systems both performance and final water quality are

dependent on the algae present in the system. Algae are func-

tionally beneficial, providing oxygen to support other biologi-

cal responses, and the algae-carbonate reactions are the basis

for the effective nitrogen removal in the ponds. However,

algae can be difficult to remove from the effluent. As a result,

the stringent limits for suspended solids in the effluent can not

be met. The design features and performance expectations for

natural aquatic treatment units are summarized in Table 2.

Aerobic ponds Aerobic ponds, also called high rate aero-

bic ponds, maintain dissolved oxygen (DO) throughout their

entire depth. They are usually 30 to 45 cm deep, allowing

light to penetrate to the full depth.^3 Mixing is often pro-

vided to expose all algae to sunlight and to avoid deposi-

tion. As a result, formation of anaerobic sludge layer can be

reduced. Oxygen is provided by photosynthesis and surface

reaeration. Because of the presence of sufficient dissolved

oxygen, aerobic bacteria can stabilize the waste. Detention

time is short, usually 3 to 5 days. Aerobic ponds are lim-

ited to warm sunny climate and are used infrequently in the

United States. 2,7

TABLE 1

Design features and expected performance for terrestrial treatment units

Typical criteria

Concepts Treatment goals Climate needs Vegetation Area,b ha

Organic loading,

kg BOD 5 /ha.d

Hydraulic

loading, m/year

Effluent

characteristics, mg/L

Slow rate Secondary, or AWTa Warmer season Yes 23–280 — 0.5–6 BOD 
 2

TSS 
 1

TN 
 3 c

TP 
0.1c

FC 0 d

Rapid infiltration Secondary, or AWT,

or groundwater

recharge

None No 3–23 45–180 6–125 BOD 5

TSS 2

TN 10

TP 
 1 e

FC 10

Overland flow Secondary, nitrogen

removal

Warmer season Yes 6–40 
 95 3–20 BOD 10

TSS 10 f

TN 
 10

TP 
 6

a Advanced wastewater treatment.

b For design flow of 3800 m (^3) /d.
c Total nitrogen removal depends on type of crop and management.
d FC  Fecal coliform, number per 100 ml.
e Measured in immediate vicinity of basin, increased removal with longer travel distance.
f Total suspended solids depends in part on type of wastewater applied.
Source: Adopted in part from Refs. 2, 3, 5, and 6.
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