WASTEWATER – POND BASED TREATMENT SYSTEM

SUDIPTA SARKAR PRADEEP KUMAR

STABILIZATION PONDS

• Waste or Wastewater Stabilization Ponds (WSPs) are artificial man-made lagoons in which blackwater, greywater or faecal sludge are treated by natural occurring processes and the influence of solar light, wind, microorganisms and algae.

• These are natural or semi-engineered processes for cost-effective wastewater treatment where required degree of treatment is achieved with minimal use of mechanical, civil and electrical facilities. These are popular for small communities because of their low construction and operating costs.

• These are essentially biological treatment processes with natural facilities spread over a vast area of land.

• The effluent still contains nutrients (e.g. N and P) and is therefore appropriate for the reuse in agriculture (irrigation) or aquaculture (e.g. fish- or macrophyte ponds) but not for direct recharge in surface waters.

• The ponds can be used individually or in series of an anaerobic, facultative and aerobic (maturation) pond.

TYPES OF STABILIZATION PONDS

1. Anaerobic Ponds 2. Facultative Ponds 3. Aerobic Ponds 4. Maturation Ponds

Anaerobic Treatment Ponds are deep ponds (2 to 5 m) devoid of dissolved oxygen, where sludge is deposited on the bottom and anaerobic bacteria break down the organic matter by anaerobic digestion, releasing methane and carbon dioxide.

Anaerobic Ponds

The anaerobic pond serves to: a)Settle undigested material and non-degradable solids as bottom sludge b)Dissolve organic material c)Break down biodegradable organic material

BOD Loading: 400-3000 kg/(ha.d)

Detention Period: 5 -50 days

BOD removal efficiency: 50-85% Volume can also be calculated

considering organic loading in the range of 100 to 350 g BOD/m3/day

NH3 ≤ 80 mg NH3-N/L

Aerobic Ponds

O2

CO2, NH3

PO4, H2O

Sunlight Algae New Algae

Bacteria

Organic Matter

New Bacteria

These are shallow earthen basins for natural treatment systems using both algae and bacteria

Symbiotic relationships between algae and bacteria

Two types of aerobic ponds are used: 1. Maximize algal growth; depth is

provided 150 to 450 mm 2. Maximize oxygen concentration;

depth is up to 1.5 m

The algal production of oxygen occurs near the surface of aerobic ponds to the depth to which light can penetrate (i.e. typically up to 500 mm). Additional oxygen can be introduced by wind due to vertical mixing of the water. Oxygen is unable to be maintained at the lower layers if the pond is too deep, and the color too dark to allow light to penetrate fully

BOD5 conversion efficiency is around 95%. Soluble BOD5 are removed, but can result in suspended matters in the effluent containing algae and bacteria

Typical BOD5 loading is 40-120 kg/(ha.d). Contents need to be periodically mixed in order to avoid formation of anaerobic zones and to obtain best results.

Facultative Ponds

CO2+NH3+CH4

It functions aerobically at the surface but anaerobic conditions prevail at the bottom. The aerobic zone kept at the top is effective against release of odorous gases. It is most suited pond treatment method.

Facultative

Facultative= aerobic + anaerobic

There is a diurnal variation in the concentration of dissolved oxygen. At peak sun radiation, the pond will be mostly aerobic due to algal activity, while at sunrise the pond will be predominantly anaerobic.

Daytime pH is high, as algae use bicarbonate ions to convert to new algae. NH3 produced due to anaerobic digestion volatilizes out to atmosphere. pH above 9 also ensures killing of pathogens and E. Coli present in wastewater.

FPs help to: a) treat wastewater through sedimentation and aerobic oxidation of organic material b)Reduce odor c)Reduce some disease-causing microorganisms if pH raises d) Store residues as bottom sludge

Maturation Pond

Maturation ponds are shallower (1 to 1.5 m), with 1 m being optimal. The recommended hydraulic retention time is 15 to 20 days. If used in combination with algae and/or fish harvesting, this type of pond is also effective at removing the majority of nitrogen and phosphorus from the effluent.

These are essentially designed for pathogen removal and retaining suspended stabilized solids.

The principal mechanisms for fecal bacterial removal in facultative and maturation ponds are HRT, temperature, high pH (> 9), and high light intensity. Fecal bacteria and other pathogens die off due to the high temperature, high pH or radiation of the sun leading to solar disinfection

DESIGN OF A FACULTATIVE POND (AS PER IS 5611)

Step 3. Find out the surface area of the stabilization pond based on the modified BOD loading rate.

Step 1. Find out permissible organic loading in the system based on the latitude of the place from the table.

Step 2. Find out the modified organic loading based on the elevation of the place above MSL using the following formula

EL)10*3(1

latitudeon based Loading BOD4

Modified BOD Loading

EL is the elevation of the place from MSL, m

Step 4. Modify the surface area based on the following formula

Permissible BOD loading at different latitudes*

Latitude (N) degree

Organic loading (kg BOD5/ha.d)

36 150

32 175

28 200

24 225

20 250

16 275

12 300

8 325

*Based on MSL and 75% clear sky

Modified Surface area area surface *

10

0.03*factor) clearancesky percentage -(751

Step 2. Find out the detention period using the formula and then ideal volume using following equations:

tk

i

e eL

L1

tkL

L

i

e

11

1

Le and Li are effluent and influent BOD5, t is the detention time, k1 is substrate

removal rate constant. K1 is also temperature dependent, standard value being at 20 deg C around 0.03 to 0.2 per day. Temperature correction coefficient 1.035

For plug flow For completely mixed system

Natural and constructed systems do not exactly follow either plug flow or completely mixed systems, hence corrections in the form of dispersion numbers are to be incorporated.

d

a

d

a

d

i

e

eaea

ea

L

L

2222

2

1

.)1(.1

.4

tdka 141

Number DispersionuL

Dd

D= Axial dispersion coefficient, length2/time

u= flow velocity, length/time L= Length of flow, length

Neither plug flow nor completely mixed systems

Ideally, the system becomes completely mixed system when d= ∞ and at d= 0, system becomes a plug flow reactor. Practically for d> 3.0, it is considered a completely mixed system, for d<0.5, it is considered to approach a plug flow

Step 3. Find out the surface area and the depth. The optimal depth is 1.5 m. If the calculation shows less than 1 m depth, minimum depth to be provided is 1 m.

Step 4. calculate sludge accumulation based on the design data of accumulation rate of 0.07 cum/ person/ year. The de-sludging period is normally taken to be equal to 6-12 years. Add depth for accumulation of sludge.

Step 5. Find out the sizes of the stabilization pond. Find out the size based on the dispersion number found out. Provide a free board of 0.5 m to 1 m.

Estimated values of Dispersion Numbers D/uL at different Length to Width ratio

Ponds Approximate range of D/uL values

Typical Mixing Condition

L:W = 1:1 to 4:1 3.0-4.0 and more Completely mixed

L:W = 8:1 or more 0.2-0.6 Approaching plug flow

Two or three ponds in series

0.2-0.6 (overall) Approaching plug flow

Step 5. pathogenic bacteria reduction is found out by the following formula

n

bi

e

tkN

N

)1(

1

Kb= 2.0 at 20 deg C

Temperature correction coefficient is 1.19

n= number of ponds in series

k1.t

Percentage Remaining, (Le/Li)*100

DESIGN OF A FACULTATIVE POND Design a facultative stabilization pond to treat 5000 m3/d municipal wastewater, BOD5 230 mg/L, from a town with population of 20,000, located in Central India, latitude 22 deg N, elevation 100m above mean sea level (MSL). The average temperature in January is 18 deg C. The effluent from the pond is to be used for irrigation. Consider that de-sludging interval is equal to 6 years. Irrigation waters have a limit of BOD5 of 100 mg/L. Consider a dispersion number of 3.0 for the design purpose. Take substrate removal rate constant as 0.2/d and its temperature correction as 1.035.

Allowable organic loading rate at 22 deg N = 237.5 kg BOD5/(ha.d) (From the chart)

Modified Organic loading rate EL)*103(1

latitudeon based Loading BOD4

X

6.230100)*103(1

237.54

X

kg BOD5/(ha.d)

Surface area required for the pond 2

5

m 49874/(ha.d)BOD kg 230.6

kg/d 1150

Incoming BOD load = 5000 m3/d* 230 mg/L = 1150 kg/day

K1 at 18 deg C d/187.0)035.1(*2.0 2018

435.0230

100

i

e

L

LFrom the chart, k1t=1.4 days 5.7

187.0

4.1t

Volume of the tank = 5000 m3/day * 7.5 days = 37500 m3

Depth of the tank =

m 17.049874

8400

Provide a minimum depth of 1m

( < 1m)

Sludge accumulation rate = 0.07 cum/ person/year * 20000 = 1400 cum/year

Desludging interval is 6 years Sludge storage volume = 1400* 6 = 8400 cum

Extra Depth to be provided =

m 75.049874

37500

Add 0.5 m as freeboard Total Depth = 1 + 0.5 + 0.17 m = 1.67 m

Provide L:W = 2:1 Size of the tank = 158 m X 316 m