4. Chapter 4 Wastewater Management by WSP System

7/31/2019 4. Chapter 4 Wastewater Management by WSP System

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Wastewater management

Looking at the present situation of village Tintoda, as it has been already

stated, village has 25 % drainage facility of what is actually required.

Wastewater from the drainage is discharged in to a small lake situated inthe downtown area. No treatment is being carried out before discharging

the wastewater in to the lake. For such a condition, waste stabilization

ponds can be a cost effective solution.

Suitability:

Waste Stabilization Ponds (WSPs) are large, shallow basins in which raw

sewage is treated entirely by natural processes involving both algae and

bacteria. They are used for sewage treatment in temperate and tropical

climates, and represent one of the most cost-effective, reliable and easily-operated methods for treating domestic and industrial wastewater. Waste

stabilization ponds are very effective in the removal of faecal coliform

bacteria. Sunlight energy is the only requirement for its operation. Further,

it requires minimum supervision for daily operation, by simply cleaning the

outlets and inlet works. The temperature and duration of sunlight in tropical

countries offer an excellent opportunity for high efficiency and satisfactory

performance for this type of water-cleaning system. They are well-suited for

low-income tropical countries where conventional wastewater treatment

cannot be achieved due to the lack of a reliable energy source. Further, the

advantage of these systems, in terms of removal of pathogens, is one of

the most important reasons for its use.

Components of WSP system:

WSP systems comprise a single string of anaerobic, facultative and

maturation ponds in series, or several such series in parallel. In essence,

anaerobic and facultative ponds are designed for removal of Biochemical

Oxygen Demand (BOD), and maturation ponds for pathogen removal,

although some BOD removal also occurs in maturation ponds and some

pathogen removal in anaerobic and facultative ponds.

Anaerobic ponds

Anaerobic ponds are commonly 2 5 m deep and receive wastewater with

high organic loads (i.e., usually greater than 100 g BOD/m3.day, equivalent

to more than 3000 kg/ha.day for a depth of 3 m). They normally do not

contain dissolved oxygen or algae. In anaerobic ponds, BOD removal is

achieved by sedimentation of solids, and subsequent anaerobic digestion inthe resulting sludge. The process of anaerobic digestion is more intense at


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temperatures above 15o C. The anaerobic bacteria are usually sensitive to

pH


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photosynthetic activity of the micro-algae that grow naturally and profusely

in facultative ponds

Facultative ponds are designed for BOD removal on the basis of a relatively

low surface loading (100 400 kg BOD/ha.day), in order to allow for the

development of a healthy algal population, since the oxygen for BOD

removal by the pond bacteria is generated primarily via algal

photosynthesis. The facultative pond relies on naturally-growing algae. The

facultative ponds are usually dark-green in colour because of the algae they

contain. Motile algae (Chlamydomonas and Euglena) tend to predominate

the turbid water in facultative ponds, compared to none-motile algae

(Chlorella).

The algal concentration in the pond depends on nutrient loading,

temperature and sunlight, but is usually in the range of 500 - 2000 g chlorophyll-a/liter (Mara, 1987). Because of the photosynthetic activities of

pond algae, there is a diurnal variation in the dissolved oxygen

concentration. The dissolved oxygen concentration in the water gradually

rises after sunrise, in response to photosynthetic activity, to a maximum

level in the mid-afternoon, after which it falls to a minimum during the

night, when photosynthesis ceases and respiratory activities consume

oxygen. At peak algal activity, carbonate and bicarbonate ions react to

provide more carbon dioxide for the algae, leaving an excess of hydroxyl

ions. As a result, the pH of the water can rise to above 9, which can kill

faecal coliform. Good water mixing, which is usually facilitated by wind

within the upper water layer, ensures a uniform distribution of BOD,

dissolved oxygen, bacteria and algae, thereby leading to a better degree of

waste stabilization.

Maturation Ponds

The maturation ponds, usually 1-1.5 m deep, receive the effluent from the

facultative ponds. Their primary function is to remove excreted pathogens.

Although maturation ponds achieve only a small degree of BOD removal,

their contribution to nutrient removal also can be significant. Maturation

ponds usually show less vertical biological and physicochemical

stratification, and are well-oxygenated throughout the day. The algal

population in maturation ponds is much more diverse than that of the

facultative ponds, with non-motile genera tending to be more common. The

algal diversity generally increases from pond to pond along the series

(Mara, 1989). Although faecal bacteria are partially removed in the

facultative ponds, the size and numbers of the maturation ponds especially

determine the numbers of faecal bacteria in the final effluent. There is

some removal of solids-associated bacteria in anaerobic ponds, principally


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by sedimentation. The principal mechanisms for faecal bacterial removal in

facultative and maturation ponds are now known to be:

(a) Time and temperature;

(b) High pH (> 9); and

(c) High light intensity, combined with high dissolved oxygen concentration.

Time and temperature are the two principal parameters used in designing

maturation ponds. Faecal bacterial die-off in ponds increases with both time

and temperature (Feachem et al., 1983). High pH values (above 9) occur in

ponds, due to rapid photosynthesis by pond algae, which consumes CO2

faster than it can be replaced by bacterial respiration. As a result,

carbonate and bicarbonate ions dissociate, as follows:

2 HCO3- CO 32 + H2O + CO2 (2.9)

CO32- + H2O2 2 OH - + CO2 (2.10)

The resulting CO2 is fixed by the algae, and the hydroxyl ions accumulate,

often raising the pH to values above 10. Faecal bacteria (with the notable

exception ofVibrio cholerae) die very quickly at pH values higher than 9

(Pearson et al., 1987c). The role of high light intensity and high dissolved

oxygen concentration has recently been elucidated (Curtis et al., 1992).

Light of wavelengths between 425 700 nm can damage faecal bacteria by

being absorbed by the humic substances ubiquitous in wastewater. They

remain in an excited state sufficiently long to damage the cell. Light-

mediated die-off is completely dependent on the presence of oxygen, as

well as being enhanced at high pH values. Thus, the sun plays a three-fold

role in directly promoting faecal bacterial removal in WSP, and in increasing

the pond temperature, and more indirectly by providing the energy for

rapid algal photosynthesis. This not only raises the pond pH value above 9,

but also results in high dissolved oxygen concentrations, which are

necessary for its third role; namely, promoting photo-oxidative damage.

Design of the component units of WSP system:

Design parameters

There are four important design parameters for WSP, including

temperature, net evaporation, flow and BOD. The climate also is important

inasmuch as the processes responsible for BOD5 and fecal bacterial removal

are temperature-dependent. Further, algal photosynthesis depends on solar

insulation, itself a function of latitude and cloud cover. Cloud cover periods

are seldom a problem because the solar insulation during the day intropical and sub-tropical regions generally greatly exceeds the saturation


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light intensity of the algae in the ponds. The design temperature usually is

the mean air temperature in the coolest month (or quarter). The pond water

is usually 2-3o C warmer than the air temperature in the cool season, with

the reverse also being true.

Because the bacteria responsible for treatment are mesophilic, high

temperatures are not a problem. However, low temperatures can be since

they slow down the treatment process. In the case of the methanogenic

bacteria (crucial to anaerobic digestion), methane production virtually

ceases below temperatures of 150 C. Thus, in areas where the pond

temperature remains below 150 C for more than a couple of months of the

year, careful consideration should be given to deciding whether or not

anaerobic units are needed. Net evaporation (evaporation minus rainfall)

must be taken into account during the design of facultative and maturation

ponds, but not for anaerobic ponds (Arthur, 1976). Anaerobic pondsgenerally have a scum layer, which effectively prevents significant

evaporation.

Total nitrogen and free ammonia (NH3, rather than NH+4 + NH3) are

important in the design of wastewater-fed fishponds. Typical concentrations

of total nitrogen in raw domestic wastewater are 20-70 mg N/l, and total

ammonia (NH4+ + NH3) concentrations are 15 40 mg N/l. Faecal coliform

numbers are important if the pond effluent is to be used for unrestricted

crop irrigation or for fishpond fertilization. Grab samples of the wastewater

may be used to measure the faecal coliform concentration if wastewater

exists.

Geometry of ponds:

To avoid sludge banks forming near the inlet, generally, anaerobic

and primary facultative ponds should be rectangular, with length-to-

breadth ratios of 2-3 to 1.

The geometry of secondary facultative and maturation ponds can

have up to 10 to 1 length-to-breadth ratios to better approximateplug flow conditions.

Avoid the use of multi-inlet and/or outlet. The inlet should not

discharge centrally in the pond as this maximizes hydraulic short-

circuiting.

A single inlet and outlet should be located in diagonally opposite

corners of the pond.

To facilitate wind-induced mixing of the pond surface layers andmaximize the settlement of solids, the pond should be oriented so


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that its longest dimension (diagonal) lies in the direction of the

prevailing wind.

Although pond depth recommendations have been given, the depth

will need to be related to the site conditions such as whether there

are rock strata, or the height of the water table.

Pond width should be kept less than 24 m because of the reach

limitations of excavator and desludging machinery.

When designing the pond geometry, it is necessary to take into

account the possibilities for the access of machinery used for

desludging and emptying both sides of the ponds.

Baffles should only be used with caution. In facultative ponds, when

baffles are needed because the site geometry is such that it is notpossible to locate the inlet and outlet in diagonally opposite corners,

care must be taken in locating the baffle(s) to avoid too high a BOD

loading in the inlet zone (and consequent possible risk of odor

release).

In maturation ponds baffling is advantageous as it helps to maintain

the surface zone of high pH, which facilitates the removal of faecal

bacteria.

A 50 cm freeboard should be provided in the design. For pondsbetween 1 ha and 3 ha, the freeboard should be 0.5-1 m.

The topography may necessitate subdividing ponds into a series of

two or more parallel ponds. Furthermore, for population more than

10,000, this subdivision is even recommended so as to increase

operational flexibility.

The effluent quality and the performance of secondary facultativeponds are independent of pond geometry, at least within the range of

length to breadth ratios of 1 to 6 and within the depth range of 1 to 2

m (Mara et al 2001).

A. Estimation of water flows and BOD concentrations :

Li = 1000B/q

Where, Li is wastewater BOD (mg/l),

B is BOD contribution (g/capita.day),


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q is wastewater flow (L/ capita.day).

Derived values: B= 40 g/capita. day, q= 100 litres/capita. day

Li = 100040/100 = 400 mg/l

B. Design of WSP Units:

Anaerobic Pond:

Volume of the anaerobic pond:

The anaerobic ponds are designed on the basis of volumetric loading ( v,

g/m3/d), which is given by:

v,= L iQ/Va

Where,

Li is influent BOD (mg/l),

Q is flow rate (m3/day),

Va is anaerobic pond volume (m3).

Considering, volumetric loading as 350 g/m3/d, (Temp > 25 C)

Li = 400 mg/l = (400 1/1000 g ) (1/1000 m3) = 400 g/m3

Q = (100/1000) 7000 = 700 m3/d

Va = (400 700)/350 = 800 m3

Hence, volume of the anaerobic pond is determined as 800 m3

Hydraulic Retention Time:

a = V a/Q

a = 800/700 = 1.15 days

Area of the anaerobic pond

Assuming depth as 2.5 m,

Aa= 800/2.5 = 320 m2

Facultative pond:

Facultative pond area :

Af= 10LiQ/s


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Where S is design surface loading kg BOD/ha day at 30 C temperature.

s = 350 (1.107-0.002T)T-25 = 581 kg BOD/ha day

Af= (10 120-700)/516.7 = 1445.78 m2

Hydraulic Retention time:

f = 2 AfD/(2Qi 0.001Afe)

f = 21625.71.5/(2700 0.0011625.75) = 3.11 days

Hence, considering 4 days of retention time.,

The cumulative filtered BOD removal in the anaerobic and facultative ponds

is 90% for T> 20 C, so the facultative pond effluent has a filtered BOD of

(0.1 400), i.e. 40 mg/l, which is not suitable for river discharge but can bedischarged on land for irrigation purpose.

Maturation Pond:

Hydraulic Retention time:

m = {[Ni/Ne(1 + kT a)(1 + kT f)] 1/n 1}/kT

Ni = Influent faecal coliform per 100 ml = 5 107

Ne = Effluent faecal coliform per 100 ml = 1000

kT = first order rate constant for FC removal, d-1 kT = 2.6 (1.19)T-20

a, f = HRT for anaerobic pond and facultative pond

n= Number of maturation ponds

m = {[Ni/Ne(1 + kT a)(1 + kT f)] 1/n 1}/kT = 4.6 days = 5 days

Area of the maturation pond:

Qi = Q 0.001 Afe = 700 - .0011445.785 = 692.8 m3/day

Am = 2Qi m / (2 D + 0.001e m) = 2290.24 m 2 considering D=1.5 m

Design considerations:

Finney and Middlebrooks (1980) stated that consistent prediction of pond

performance by any design method without accurate projections of

hydraulic residence time is impossible. Shilton (2001) presented an

extensive study on the hydraulics of stabilization ponds. Twenty

experimental configurations were tested in the laboratory and ten of theseexperimental cases were mathematically modeled and had good


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agreement with the experimental work. Shilton and Harrison (2003) then

introduced broad and informative guidelines for hydraulic design of WSP to

"help fill the knowledge gap in the pond hydraulics area". Although

engineering judgment is always required, and the current understanding of

ponds hydraulics is still limited, the following observations were proven tobe useful for the purpose of improving WSP hydraulics, and consequently

ameliorating WSP design, performance and efficiency:

Short-circuiting (when water enters and leaves the pond in a very

short time) shall be avoided as it results in a large reduction in the

discharge quality.

Influent should be mixed into the main body of the pond to avoid

localized overloading, taking into consideration not to create short-

circuiting.

The solids deposition within the pond occurs as a result of the flow,

rather than the flow being redirected as a result of the solids.

Inlet position and type has a significant impact on treatment

efficiency in ponds.

Dropping inlets from horizontal pipes above the water have similar

behavior as submerged horizontal inlets.

For high-load wastewaters, horizontal inlets may be needed to mixwastewater into the pond. Consider baffles and outlet positioning to

avoid short-circuiting problems.

For low-load wastewaters, consider a manifold or baffled vertical inlet

but only after consideration of wind influences.

Inlet positioning has a major influence on the flow pattern.

Designers need to consider the effect of inlet position in conjunction

with outlet position and pond shape/baffles. A pond should maintain a similar and reasonably well defined flow

pattern through a range of different flow rates.

Outlets should be placed out of the main flow path of the incoming

wastewater (close into a corner).

Final outlet positioning can be selected after the inlet position/type

and pond/ baffling have been designed.

Outlet manifolds are not recommended.


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Long evenly spaced baffles improve pond performance. Baffles of

70% width gave superior performance compared with 50% and 90%

width.

Horizontal baffles were found to be more efficient than vertical

baffles.

Longitudinal baffling was found to be no more efficient than

transverse baffling.

Localizing baffles close to horizontal (but not other types!) inlets is

generally effective.

A minimum of two baffles in a pond is recommended. A further

improvement was achieved using four baffles and this extra cost may

be warranted in some cases. Based on Shilton and Harrison study(2003), more than four baffles would not be recommended.

Traditional thinking that, in a long narrow pond, the influent simply

flows slowly from one end to the other is not necessarily correct

except at very high length to width ratio.

Baffles that shield the outlet are beneficial.

A diversion channel should be build around the pond (the topside) to

divert storm water runoff coming from adjacent areas.

PVC pipe, of at least 100 mm diameter is recommended for carrying

effluent to the pond and between ponds.

All ponds should be surrounded by a fence for public safety and

health protection.

Standards:

Note

It is sometimes asked what is the lowest concentration of BOD at which

WSP can operate. Generally speaking, WSP can operate satisfactorily at any

level of BOD, although it is worth noting the following three points:

(a) as noted in section anaerobic ponds should have a minimum retentiontime of 1 day; however, if the resulting volumetric BOD loading is


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g/m3d, then anaerobic ponds should not be used as there is essentially no

experience of their satisfactory performance at lower loadings;

(b) Facultative ponds should have a minimum retention time of 4 days at

design temperatures above 20C and 5 days at lower temperatures; the

resulting BOD loading may be much less than that permitted by equation if

the wastewater BOD is very low, but this does not matter the algal

population will adjust accordingly and the nominally facultative pond will

function algologically more as a maturation pond, but treatment efficiency

will not be seriously impeded; and

(c) if the wastewater BOD is below, or only slightly above, the CPCB effluent

discharge standard of 30 mg/l (which might be due to excessive infiltration

in the sewer system, for example), then probably no treatment would be

required.

Geometry of the pond


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4. Chapter 4 Wastewater Management by WSP System