Sludge formation in pond bottom
In the intensive shrimp culture ponds are heavily aerated and it is common practice to position aerators in the comers of ponds to create a circular motion of water. If excessive aeration is used, bottom material is eroded from inside slopes of levees and around the periphery of the pond bottom. Waste products are being produced continuously during shrimp culture in a mixture of gases, liquids, semi-solid and solid forms. Some of these waste materials are removed through discharge and some settles out on the pond bottom which becomes semisolid or solid waste. The suspended materials then settle in the central areas of ponds as sediment where water currents are weak. These organic matters are formed largely from the residue of pond inputs such as uneaten feed, shrimp faeces, and plankton, biological wastes from the shrimp and other organisms and eroded soil.
The sediment in a strip around the periphery of ponds was hard, brown and sandy. In the other areas, fine black sediment of varying thickness overlaid coarser sediment. The waste material either collects on the pond bottom or in ponds with porous soils it may penetrate into the pond soil.
The major characteristic of sludge formation is based upon design and type of pond, culture system, pond management regime, and pond inputs. Based on its nature and source, sludge have higher value of organic matter, total nitrogen, COD, BOD and phosphorous than normal soil.
Effects of sludge
Effect of pond waste on shrimp culture
Growth, survival of the shrimp and quality of pond water is greatly affected with pond waste. Removal of sludge frequently from the pond bottom, the organic nutrients reduces from water and it affects the growth of phytoplankton with pond productivity. The growth of various shrimp fries has shown a negative correlation with ammonium and sulfide concentrations of sediment. The shrimp survival and production have been affected in ponds on moderate removal of sludge during culture period.
High sludge in pond bottom may lead to accumulation in pond that not only increases sediment oxygen demand but creates anaerobic conditions resulting in undesirable gas production such as hydrogen sulfide. Large volume of accumulated shrimp pond waste will increase oxygen demand and cause oxygen depletion on the bottom that stress shrimp and more susceptible to disease. The undesirable gases produced from pond waste can affect the appetite of shrimp, increasing feed conversion ratio and leading to deterioration of water quality.
In aquaculture ponds, the rate of oxygen consumption by the mud increases during a grow-out period because inputs of organic matter tend to increase. In intensive shrimp culture ponds, feeding increases organic matter in pond waste leading to higher oxygen demand at the mud-water interface and may cause production of hydrogen sulfide gas. In order to avoid these unfavorable conditions in pond environment, pond waste has to be managed by removing at certain period of time.
Effect of Pond waste on Environment
Pond waste produces negative, neutral and positive impacts on environment. The degree of impact intensity and its consequences is largely dependent upon sludge management practiced during culture operation and post culture period. The sludge from the shrimp farming industry on environment can be classified into three divisions.
- Impact on coastal water quality and hydrology.
- Impact on aquatic organisms
- Impact on mangrove and terrestrial vegetation.
Impact on coastal water
The load of effluent exiting from the shrimp farms normally exceeds the carrying capacity of the receiving water bodies. Nutrients in effluent promote the eutrophication to produce plankton in adjacent estuary or nearshore ecosystems along with sedimentation. However, nutrients from the effluent and plankton serve as the potential food source for many species of coastal fish and invertebrates.
Impact on aquatic organisms
Pond waste contains numerous suspended solid particles which cause turbidity in the receiving water. Turbidity in water reduces light penetration, thereby decreasing photosynthetic activity and dissolved oxygen level causing stress to aquatic organisms especially bottom dwelling animals. Presence of nitrogen and phosphorous in high amounts contributes to eutrophication in ponds.
Impact on mangrove and terrestrial vegetation
Mangrove absorbs excess nutrients and other pollutants from entering into the seawater. The export of the large amounts of organic matter in the form of detritus by tidal current support the productivity of the adjacent coastal ecosystem. Carbon fixation is a process that transforms the low energy carbon dioxide into high energy compounds. Applications of pond waste to terrestrial vegetation show negative effect as the concentration level of salts are high. The characteristic of shrimp pond waste differs with pond water quality, rainfall and pond inputs. Higher salinity in pond water results in higher content of salts in shrimp pond waste.
Pond waste management
Effective pond waste management has to be carried out in two separate phase production management and post harvest management. A complete pond waste management strategy combines four approaches: Control, treatment, disposal and reuse. Management techniques are different from one farm to another depending upon personal preference, affordability, suitability and pond management techniques.
Pond waste management during culture operation
During culture period, the different techniques are carried out to manage the pond waste depending upon culture situation, pond and environmental condition and resources availability. Three of the most useful approaches to pond waste management are ‘remain’,‘remove’ and ‘re-suspend’.
The ‘remain’ management technique refers to accumulation of pond waste within the pond where it may produce least negative effects to shrimp population. The pond waste is usually concentrated in the middle of culture pond in order to create larger clean space for the shrimp to inhabit around the edges. The different techniques are carried out in shrimp culture pond such as different aeration equipment, shallow ditches, the control approach, managing feed and pond erosion, application of chemicals such as oxidants, use of probiotics, use of biodegradable compounds, bottom aeration etc.
The ‘remove’ management technique implies removal of pond waste from grow-out ponds during the culture period with aiming to create more clean space for shrimp. However, ponds with high nutrient availability and high waste loading rate usually have continuous deposition of pond waste at a high rate. In this case, pond waste that deposited in the middle of the pond is usually removed completely as new pond waste keeps moving inward so nutrients are available for maintaining plankton growth.
Removal techniques of different devices are being used to remove pond waste from pond during the culture operation such as use the central drain system, electric or mechanical suction pumps rotational pond waste removal devices etc.
Small particles of resuspended pond waste increase the surface area available for bacterial attachment leading to faster breakdown. Utilization of this technique causes high BOD in the water column requiring more dissolved oxygen to balance the oxygen budget in the pond. Higher suspended solid concentrations also reduce light penetration, which is crucial for photosynthesis. .
The bottom aeration is provided from the beginning most wastes are digested aerobically and only a small portion of waste is deposited on the bottom after the harvest. The extensive aeration provided on the pond bottom, oxidizes waste particles and allows aerobic decomposition and thus produces a beneficial outcome.
Post-culture pond waste management
The disposed pond waste is usually settled or sundried naturally and its salinity thoroughly reduced by rain. Proper post culture pond waste management procedure can be divided into four phases, control, treatment, disposal and reuse/utilization.
The four management phases carried out after harvest are in sequential order and its level of management, in terms of environmental sustainability, increases with the phase.
Control management: control phase is preventing pond waste effects on shrimp culture itself and minimizing the discharge of untreated pond waste into open environments. This phase includes proper planning of pond waste treatment and disposal activities on farm. Pond waste is gathered at one place and at least confined to the on-farm environment even if the waste is untreated.
Treatment management: the treatment aims to reduce the volume, toxicity of pond waste and make it useful for other purposes. The required treatment of pond waste varies with its characteristics and dependent upon pond management technique, type of pond and pond inputs.
Disposal management: proper planning and the provision of an area for discharging pond waste in an environmentally friendly and safe manner. Implementation of this phase greatly improves environmental quality and reduces health risks.
Utilization management: the ultimate goal of pond waste management is ‘utilization’ including recycling waste products and increasing productivity of other production sectors such as agriculture. Pond waste is even useful in shrimp culture as a nutrient source when culturing phytoplankton.
Extent of impact depends on following factors
• Type of water exchange and frequency
• Intensity of culture system (density & feeding)
• Characteristics of water bodies that will receive effluents
• Water circulation (closed or open system)
• Existing water quality
Guidelines for Shrimp Pond Waste Management during culture operation
The following should be observed as a general guideline for pond waste management. Although these are mainly for the farm operators and owners to follow it still needs assistance of related government agencies.
- All production farms (regardless of size or production capacity) should have an area for disposing waste before planning any production activities.
- Waste disposal area should be adjusted after every crop in line with waste production level, local environmental conditions and government requirements.
- Farms that use ‘remain’ management approaches should have additional management systems to lower pond waste volume and improve quality of pond waste while in operation.
- Farms that use ‘remove’ management approaches should have a proper waste management system before disposing out of farm environment.
- Use of chemicals and drugs to manage pond waste should be avoided where possible.
Guidelines for Post Culture Pond Waste Management
- Shrimp Pond Waste should not be discharged to outside environment.
- There should be proper and sufficient disposal area for Shrimp pond waste on farm.
- Primary treatment such as sedimentation and sun drying should be performed before the waste is disposed off.
- A certain degree of treatment should be applied to SPW before the disposal based on pond waste condition: Quality, volume and especially if the pond had received some probiotic and antibiotic treatment or if the pond had disease problems.