4.3.3 Waste Stabilization Pond

Categories: 4.3 Secondary Treatment
Time of issue: 2022-04-28 18:31:08
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Waste Stabilization Pond (WSP)

Waste stabilization ponds are large, constructed basins/depressions filled with wastewater that involve natural treatment processes to remove organic matters, nutrients, and pathogens in wastewater. The treatment process is a complex symbiosis of bacteria and algae.

Figure 4.10 Schematic diagram of waste stabilization ponds

(Source: Tilley et al, 2014)

Treatment Process:

There are three types of waste stabilization ponds:

Anaerobic ponds: They have a depth ranges from 3 to 5 meters with a small surface area. The depth decreases the dissolved oxygen levels in water leading to anaerobic conditions. Suspended solids settle down at the bottom of the pond, and anaerobic digestion occurs to degrade organic matters and nutrients in the wastewater. The retention time is 1 to 7 days.

Facultative ponds: Also known as oxidation or photosynthetic pond, they have a depth ranges from 1 to 2.5 meters. Large surface area of the pond allows atmospheric oxygen to dissolve in and sunlight radiation to penetrate the water. Photosynthesis by algae also increases the dissolved oxygen in water. Aerobic degradation of organic matters and nutrients occurs in the upper layer, aerobic and anaerobic degradation takes place in the middle layer, and sludge at the bottom undergoes anaerobic digestion. The retention time is 5 to 30 days.

Maturation ponds: They have a depth ranges from 0.5-1.5 meters. The shallow depth allows sunlight penetrating to the bottom of the pond for photosynthetic activities that release oxygen. Great surface area of the pond enables more atmospheric oxygen dissolve into water. Pathogens naturally die in the pond. The retention time is 3 to 5 days.

The three types of WSPs are usually arranged in sequence to achieve high removal efficiency of organic matters, nutrients, and pathogens in wastewater. Typically, wastewater flows through the anaerobic pond followed by the facultative pond, and finally enters the maturation pond before discharge with a total retention time of between 10 and 50 days. In essence, anaerobic and facultative ponds are designed for BOD removal and maturation ponds for pathogen removal, although some BOD removal occurs in maturation ponds and some pathogen removal in anaerobic and facultative ponds. In many instances only anaerobic and facultative ponds are required. In general, maturation ponds are required only when wastewater with low organic loading is to be treated prior to surface water discharge and when the treated wastewater is to be used for irrigation for vegetable crops.

Water Quality of Influent:

Type of CW	BOD₅ (mg/L)	COD_cr (mg/L)	SS (mg/L)	NH₃-N (mg/L)	TP (mg/L)
Anaerobic pond	≤400	≤900	≤200	≤75	≤8.0
Facultative pond	≤200	≤500	≤150	≤35	≤6.0
Maturation pond	≤60	≤150	≤100	≤20	≤4.0

Source: Technical Specification for Natural Treatment of Wastewater Engineering (CJJT 54-2017).

Treatment Efficiency:

Pollutant/Parameter	Removal efficiency
Pollutant/Parameter	Anaerobic pond	Facultative pond	Maturation pond
BOD	50-70%	60%	20%
Suspended solid	50-70%	-	-
N	50-70% (the treatment system as a whole)
P	50-70% (the treatment system as a whole)
Total Coliforms	-	-	90% (t>15℃)
Note: The treatment efficiency varies depending on temperature (seasons) and DO. Variation of hydraulic loading and organic loading will not impact the treatment efficiency. Due to existence of algae, the SS in effluent may be relatively high.

Source: SUEZ Water Treatment Handbook, 2020.

Design Criteria:

The pond shall be sited at a safe distance from and downwind of residential area to avoid potential odor impacts.
Anaerobic ponds shall be configurated in parallel and aerobic ponds shall be configurated in series.
The slope of site shall not exceed 2%. If the slope is larger than 2%, stepped configuration can be applied to achieve required water depth.
The length to width ratio shall be less than 3:1.
The designed depth of sludge layer shall be at least 0.5 m for anaerobic pond and 0.2 m for facultative pond and maturation pond.
At least 0.5 m of freeboard should be maintained when the pond water level is full.
In cold region, the thickness of ice cover during winter shall be taken into consideration for pond depth design.
A liner should be designed for the ponds to prevent leaching of wastewater into surrounding soil or groundwater. Lining must be placed on a stable soil foundation or structure.
A continuous underdrain of perforated piping or other configuration to collect groundwater below the lining that operates at atmospheric pressure should be put in place.
For ease of maintenance and flexibility of operation, at least two trains of ponds in parallel shall be designed.
The inlet shall be set 0.6-1.0 m above the pond bottom and at least 0.3 m beneath the water surface.
Multiple inlet arrangements are preferred, and preferably by means of a long splitter box with multiple outlets large enough to avoid plugging by influent solids.
To minimize short circuiting due to wind, the pond inlet-outlet axis should be aligned perpendicular to the prevailing wind direction. If for some reason the inlet-outlet axis cannot be oriented properly, baffling can be used to control, to some extent, the wind-induced circulation.
Macroohytes, such as duckweek and reed, shall not be planted in facultative and aerobic ponds to ensure sunlight can penetrate the water for photosynthesis.
Berm. A protective berm should be constructed around the pond using the excavated material to protect the ponds from runoff and erosion. The clay soil making up the berm should be well compacted in 30 cm lifts and void of all topsoil and organics. The minimum berm height standard is intended to ensure surface water does not flow into the pond.
Fencing. A fence should be installed to ensure that people and animals stay out of the area and that garbage does not enter the ponds. The fence should be at least 1.2 m tall and have an access from one side by a locking gate.
Installation of mixers or aerators can help increase the dissolved oxygen in water.

Key Design Parameter:

The design parameters shall be determined on the basis of local climate conditions, experimental data analysis, or operation experiences from other similar waste stabilization ponds. The following table listed reference data for key parameters for waste stabilization ponds at different temperature.

Table 4.4 Key design parameters for waste stabilization pond

Item		BOD₅ area loading rate (g/(m²Ÿd) (volumetric loading rate for anaerobic pond)			Effective water depth (m)	Hydraulic retention time (d)			Treatment efficiency (%)
Item		Zone I	Zone II	Zone III	Effective water depth (m)	Zone I	Zone II	Zone III
Anaerobic pond		4-8	7-11	10-15	3-6	≥8	≥6	≥4	30-60
Facultative pond		2.5-5	4.5-6.5	6-8	1.5-3	≥30	≥20	≥10	50-75
Aerobic pond	Conventional treatment	1-2	1.5-2.5	2-3	0.5-1.5	≥30	≥20	≥10	60-85
Aerobic pond	Advanced treatment	0.3-0.6	0.5-0.8	0.7-1	0.5-1.5	≥30	≥20	≥10	30-50

Note: Zone I: annual average temperature＜8℃; Zone II: annual average temperature between 8℃ and 16℃; Zone III: annual average temperature >16℃.

Source: Technical Specification for Natural Treatment of Wastewater Engineering (CJJT 54-2017).

Operation and Maintenance:

Spring or early summer is the best time for startup to avoid low temperatures and possible freezing. Fill primary cell(s) with water from a river or municipal system, if available, to the 0.6 m level. Begin to add the wastewater, keeping the pH above 9.5 and checking the DO daily. Algal blooms should appear in 7-14 days. A good biological community will be established in about 60 days or less. The color will be a definite green, not blue or yellow-green. If is it necessary to start in late fall or winter, the water level should be brought to 0.75 - 1 m and not discharged until late spring.
Do not require skilled operator to manage the system.
Regular removal of sludge every 2-5 years in anaerobic ponds, and 15 to 25 years in facultative ponds. The sludge, usually contains high concentration of pathogens, should be treated or managed appropriately to limit human exposure.
Regular removal of scum, excessive algae and large aquatic plants on the pond surface to avoid breeding of mosquitoes/flies and promote sunlight penetration.
Regular inspection on the pipes, weirs, and other hydraulic structures to avoid blocking.
Protect anaerobic pond from cold temperature. The temperature to ensure anaerobic digestion process shall be above 15 ℃ and temperature over 25 ℃ is more preferable.
As the treatment efficiency is not impacted by hydraulic loading and organic loading, the stabilization ponds have high impact resistance and buffering capacity. Control of retention time, hydraulic circulation and adding of biological activator are the main measures to adjust the pond operation.

Capital Cost and O&M Cost:

The capital cost depends on the land price, topography, and soil conditions. The primary costs are the costs of excavation and lining of the pond. It is recommended to construct the pond at low-lying area or by reconstruction of existing water ponds in villages. If the soil permeability is low, in situ compaction can be applied to prevent seepage.
The operation cost is minimal as the system does not involve mechanical equipment and energy consumption. No skilled operator is required to management the system.
The sludge removal process is laborious and expensive.

Applicability

A good option for rural communities where large and open land is available, cheap and away from households.
Anaerobic pond is appropriate in areas with warm climate.
Maturation pond is appropriate when high removal efficiency of pathogens is required.