The Aquaculture Authority has made it mandatory that all shrimp farms of 5.0 hectare Water spread area and above located within the CRZ and 10 hectares water spread area and above located outside CRZ should have an effluent treatment system (ETS) or effluent treatment ponds/facility. Establishment of such a system is necessary to bring the shrimp farm waste water within the prescribed standards and mitigate any adverse impact on the ecology of the open waters

An effluent treatment system consisting of settlement pond (SP), bio-ponds (BP) and aeration pond (AP) is proposed for the shrimp farms practicing improved traditional and extensive methods of farming. BY incorporation of the ETS facility, the farm waste water is expected to be as good in quality as that of intake water. Quality-wise, the treated wastewater would also be suitable and ideal for re-circulation within the farm, making the farming practice conform to the zero discharge norms. However, such a system (i.e. re-circulation system) would need the establishment of a reservoir pond of suitable size.

Effluent Treatment System-Design & Lay out
Basic considerations:

 The characteristic features of the shrimp farm waste water that have been taken into consideration for designing the ETS are as follows:

• Shrimp aquaculture waste water/ discharge during normal operating conditions (i.e. during culture period) is high in volume, but relatively dilute in nature. When ponds are aerated (farms adopting improved traditional and extensive methods of farming), discharges from the ponds usually contain adequate oxygen for aquatic life and diluted concentrations of nitrogen, phosphorous and organic matter;

• The shrimp pond water quality tends to deteriorate through the grow-out period, as feeding rate increases with shrimp size and biomass. Thus, the highest quantity and poorest quality of wastewater (in terms of nutrient load, total ammonia and ionised ammonia and total suspended solid) are found just before harvest time, when shrimp biomass is at the maximum;

• The shrimp pond water quality tends to deteriorate through the grow-out period, as feeding rate increases with shrimp size and biomass. Thus, the highest quantity and poorest quality of wastewater (in terms of nutrient load, total ammonia and ionised ammonia and total suspended solid) are found just before harvest time, when shrimp biomass is at the maximum;Wastewater discharge during harvest (especially the last 5 cms drainage) is usually the most important contributor to overall waste water loading, comprising over 75% of the total load;

• Stocking densities and management practices largely influence the quality of the discharge. In areas where the intake water quality is below the desired standards, a re-circulation system can be resorted to using a storage reservoir in combination with the ETS. This would help in maintaining the water quality and animal health standards.  

 Lay-out Plan

The layout plan of the ETS is depicted in Figure 1. As per norms, 10 per cent of the cultivable area should be assigned for the ETS. For example, for a farm of 5.0 hectare water spread area, 0.77 hectare land area or approximately 0.50 hectare water spread area (actual operational area) will be required for construction of the ETS. For farms more than 5.0 hectares, the area under ETS will also proportionately increase (e.g. for a 6.0 ha. farm area, 0.6 hectare under ETS; for a 10 hectare farm area 1.0 hectare under ETS and so on). The size of the settlement pond, bio-pond and aeration pond has also been suggested taking in to consideration the optimum production level of 2.0 tonnes/ hectare/ culture and specific water management practices. Water exchange schedule to be followed for operating the system is shown in Table 3. The schedule is based on the availability of a reservoir of suitable size for storage and treatment of water for initial filling of the ponds, topping up of water level during the first two months of rearing and limited water exchange during the third and fourth months of rearing.

Even though, a modular design of ETS is shown, the design and lay-out may be suitably altered taking into consideration the location and shape of the land available for such purpose.

Water Exchange Schedule

Water exchange schedule for the farms practicing recirculation of water is given inn Table 3. 

A. Month – 3
Volume of water exchange = 10000 cu.m
(once in 15 days) 
Volume of water/ day = 10000/15 = 670 cu.m /day

Retention time available
a) Sedimentation pond = 2142 cu.m / 670 = 3.20 day
b) Bio-ponds = 1620 cu.m /670 =2.40 day
c) Aeration pond = 1153 cu.m / 670 = 1.74 day

B. Month-4
Volume of water =10000 cu.m
(Once in 10 days) 
Volume of water/day =10 000/10 =1000 cu.m /day

Retention time available
a. Sedimentation pond =2142 cu.m / 1000 = 2.14 day
b. Bio-ponds =1620 cu.m / 1000 = 1.62 day
c. Aeration pond =1163 cum./ 1000= 1.16 day

C. Harvest
The supernatant water from the culture ponds up to a depth of 30 cm is drained starting from 10 days prior to the day of harvest.
Volume of water/day =50 000x 0.7/ 10=3500 cu.m/day
Although this supernatant water will carry practically little sediment load, a retention period of 15 hours, 11 hours and 8 hours will be required in the sedimentation, bio and aeration ponds respectively.

Description of Various Ponds and their Use
Settlement pond/ Sedimentation pond
A settlement / sedimentation pond is basically used to remove suspended solids from the wastewater flow. Shrimp farm suspended solid wastes under normal operating conditions (during culture as opposed to harvest) are primarily composed of living plankton cells, feed material and other organic material, which do not easily settle down. Sedimentation tank can trap only 5 to 10 percent of such suspended solids. A retention time of one hour is sufficient to trap the material, which can settle down. Thus settlement pond is less effective in trapping the solid contents of the wastewater discharge during the course of culture. However, settlement tanks are effective in trapping suspended solids during the harvest, when solid loads are far higher and particulate matter is more dense. Studies have shown that 90% of the solids in the harvest discharge settle in sedimentation ponds. Thus the sedimentation ponds prevent the release of most polluting organic matter that is discharged at the time of harvest (last 5 to 10 cm water) to the environment.

Bio-Ponds or Biological Treatment Ponds
Biological treatment aims at using plants and animals to reduce nutrient load and particulate matter in the shrimp farm discharge. Farm discharge after the treatment in settlement and bio-ponds can be readily used for re-circulation to ponds for farming operation 
Various options available for biological treatment of farm discharge are as follows:

• Sea weeds/water weeds to reduce nutrient (N and P) level,

• Molluscs to reduce suspended particulate matter and

• Fish to transform the phytoplankton into organic matter

Biological treatment can only be used to treat operational farm wastewater i.e. during culture period, as the wastewater during harvest time is biologically unsuitable in its direct form, unless diluted. However, the harvest wastewater if allowed to remain in the settlement pond for requisite duration can be treated in the bio-pond.

Various species of weeds and animals available for biological treatment (bio-remediation), their usefulness and the constraints in using them are given in Table 4.

Based on the type of culture (i.e. production/ ha) and other features of the site, the optimum number/quantity of weeds/ molluscs/ fishes required to treat the wastewater effectively is to be arrived at.

Aeration Pond
Aeration helps to increase the dissolved oxygen levels of water before it is pumped for re-circulation. Besides, it also helps to oxidize any left over ammonia and organic matter in the water that comes out of the bio-pond

Construction of ETS
Dimensions of a model unit

Dimensions of a model unit of ETS for a farm of 5.0 hectare water spread area are provided in Table 5.

Construction
Aquaculture Authority does not permit construction of shrimp farms in sandy soil as water from such farms will seed out. However when an ETS is planned, soil permeability of the ETS site has to be cheeked and if percolation of water is suspected either the site is abandoned or a clay lining (or core of clay) should be provided to prevent seepage of effluent. As an illustration, out of 7 750 sq.m. of land area, the ETS ponds with the following dimensions are suggested.

Sedimentation/ Settlement Pond
One pond of 90 x 24 m x 1.2 m holding a volume of 2 142 cu.m water is necessary for the system. Earthen ponds will hold good. The pond should have at least five baffle walls to allow the waste to move slowly for a longer distance (circuitous), enabling the settlement of solid waste material.