Water Treatment In Aquaculture

Water treatment in aquaculture – all types of aquaculture have in common that the water quality is essential for fish farmers to achieve a rich harvest
of healthy and well-grown fish.  Implementing reliable water quality management is a powerful measure for lowering the risk of recurrent fish mortality. Directly or indirectly, water is the major influencing factor for the well-being of the fish.  The metabolic activity involved in converting fish feed to fish flesh produces waste products. These consist of suspended solids and dissolved nutrients. Suspended solids amount to approximately 25% of the feed used, on a dry matter basis.

Removal of suspended solids

Removal of suspended solids from flow-through fish farms has different requirements from other water treatment processes such as those handling sewage, pig farm effluent or dirty water from a vegetable processing plant: concentrations of pollutants in fish farm effluent are relatively low, and flow volumes are relatively high. These large water flow volumes require a careful choice of systems to control costs.

Origins of suspended solids

In a fish farm, suspended solids and dissolved nutrients originate from, uneaten feed, fish metabolism, solids carried into the farm with the flow from the external water source and growth of micro-algae and bacteria.  Factors affecting the production of suspended solids in a fish farm is affected by a range of factors, including, rate of feeding, feeding methods, water exchange rate – Tank hydrology and fish stocking density. The amount of uneaten feed can be reduced by a careful feeding regime that provides the correct amount of feed, at the time the fish require it, and by husbandry that provides water quality suitable for feed conversion. Substantial improvements in feed conversion ratios, and reductions in the faeces generated, have been achieved by improving fish diets; for example, by using ingredients with high digestibility and diets that are matched to the requirements of the fish species. Selection of feed ingredients can influence the physical stability of faeces on their journey to the point where they are removed by sedimentation or filtration.
Intensification of aquaculture practice, in response to market pressures, has resulted in farms growing more fish and generating significant volumes of suspended solids. As fish farm units grow larger quantities of fish, separation of solids from the water flow before discharge from the site becomes more important. This is particularly true where several fish farms are located close together on one river or, in the case of marine farms, where farms discharge close to one another into the sea. The need for treatment is accentuated where the rate of dilution of the effluent by the residual river flow or marine tides is low.

Choice of Systems to reduce the content of Suspended Solids in farm effluent
There are two methods for reducing suspended solids in fish farm effluent, each of which removes suspended solids from suspension:
Sedimentation – uses gravitational settlement systems of differing complexity
• Mechanical filtration – uses energy and filtration meshes dimensioned to trap solids.

Outside influences will impact on the choice of method used to remove suspended solids. Differences in site-specific situations, including farm location and water quality.

Sedimentation Systems

Before considering gravitational sedimentation methods, it is essential to understand the main principle driving the settlement of a solid in water. Gravitational sedimentation uses the force of gravity to extract particles from a fluid. Differences in density between the particles and the fluid cause the particles to travel downward in a quiescent or slowly moving liquid.

The specific gravity of fish faeces is close to that of water, and therefore the rate of their sedimentation is low. In contrast, minerals such as sand have a high specific gravity and therefore settle more quickly. Sedimentation rate depends on the characteristics of the material being settled (including their size), and on the velocity and turbulence of the water in which the particles are suspended. Sedimentation rate is measured in centimetres per second (cm/s). In aquaculture, a favourable settlement speed is 1 cm/s. Most uneaten feed and faeces are separable by sedimentation.

Sedimentation of suspended solids is made more difficult by degradation of the feed or faeces ‘pellet’ as it travels from the fish through the fish-holding area to the sedimentation basin. Water turbulence, created by the speed of water flow and the swimming action of the fish, causes the faeces to be held in suspension and to be progressively abraded and broken down into smaller particle sizes. Very small particles become ‘non-settling solids. This degradation of faeces into smaller particles, when combined with time exposure in the water, leads to a portion of the nutrients contained in the solids becoming dissolved. Fish farm design should therefore aim to trap and remove suspended solids as early as possible after being deposited by the fish, to reduce this degradation process.

Sedimentation can be achieved by the following methods and structures:
• Simple sedimentation using a large area – ponds or basins
• Centrifugal concentrators – hydro clones or cones

Simple sedimentation is achieved by structures, variously called sedimentation, or settling, ponds and basins, that make use of the settlement characteristics of the solids.  The method relies on a large area to slow the speed of flow, thus allowing time for the solids to settle.

Lamellar settlement tanks achieve settlement of solids in a restricted land area, by more sophisticated parallel and inclined barriers. They are complicated, expensive to construct and difficult to clean. They are rarely used for fish farm effluent treatment.

Optimal conditions must be achieved for each of the following key efficiency factors in order to achieve the best filtration result:
• Design of fish tanks or ponds
• Mechanical degradation of particles in the system
• Feed quality and feed management

Treatment with coagulants and flocculants

Coagulants and flocculants are chemicals that have properties that make them useful, when added to the sludge, in the further concentration of solids. Coagulants and flocculants act on material in the sludge to create (in the case of coagulants) larger molecules or (in the case of flocculants) even larger particles. Since these added chemicals become part of the material that will later be disposed of, care must be taken in their choice to ensure that their use and subsequent products are innocuous and comply with any regulations.

Coagulants are chemicals that promote molecular aggregation. Usually dissolved substances are aggregated into microscopic particles by a coagulant. The most used coagulant is iron chloride (FeCl3).

Flocculants promote the flocculation of coagulated particles into a macroscopic floc. They can be classified into two categories, namely organic and inorganic. Organic flocculants can again be divided into two subgroups: synthetic and natural.  Natural polymers, often used as flocculants, have advantages in aquaculture because of their biocompatibility and biodegradability.  Examples of natural polymers are starch from potatoes or corn; cellulose; alginic acid and guar gum. Synthetic polymers are themselves usually non-toxic, but the associated monomers often are toxic.  They are more effective than natural polymers and are preferred in drinking water treatment. Only small quantities of polymeric flocculants are required to be effective, and they are operative over a wide pH range. Suitable flocculants are found in the polyacrylic amide family.

The effective dose of coagulant is determined by experiment for each situation (specific waste characteristics and conditions) to which it is applied. This dose is added to the waste. Then the predetermined dose of flocculant is added. Coagulant and flocculant react together to create large flocs that are more easily removed from the water.

Enduramaxx manufactures a number of settlement and clarification tanks which are widely used for Water Treatment In Aquaculture as well as a range of polymer,  coagulants and flocculant mixers.  For more information, please get in touch.

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