Solids Removal Tech Info

Solids removal is a critical treatment component of influent, effluent, and recirculation treatment systems for aquaculture. In influent systems, solids removal is important to ensure water is of suitable quality for fish or for pre-filtration upstream of disinfection processes to optimize pathogen control. In effluent systems, solids removal processes are used to mitigate the impact of the aquaculture facility on the environment. In Recirculating Aquaculture Systems (RAS), removal of solid wastes on a continuous basis is necessary to prevent the accumulation in the system which will degrade water quality and reduce the efficiency of biofiltration and disinfection processes.

The main methods of solids removal include gravitational settling, flotation and filtration. Filtration can also be separated into broad categories which include media filtration and screening. Selection of solids removal technology will depend on the type, density, and size of the particles being removed, the flow rate being treated, and the specific application of the equipment. Both low head and pressurized technologies are available, the selection of which will significantly impact energy use and operating cost. Backwash method and volume may also be an important consideration in applications that require continuous flow or where water volume is limited.

Gravitational settling may be used for removal of all particles which have a density greater than water. The greater the density of the particle, the more quickly the particle will settle and the less susceptible the particle will be to re-suspension due to turbulence. Many of the particles in aquaculture waste, composed mostly of waste feed and feces, are only slightly greater in density than water and require longer settling times than are possible within most aquaculture facilities. Large settling basins are sometimes employed where space is available and freezing is not a concern, but can result in degraded water quality if the solids are retained long enough that decomposition becomes a problem. Swirl separators and Radial Flow Separators are smaller, more efficient vessels are specifically designed to minimize turbulence and maximize capture of aquaculture waste particles. These technologies are commonly used for aquaculture effluent treatment or as part of a recirculating aquaculture system and are suitable for use where operators can purge the accumulated solids routinely before decomposition can become a problem.

Flotation takes advantage of the surface tension and charges at the air/water interface. Flotation systems use fine bubbles to attach and lift fine particles to the water surface where they form a foam layer which may be skimmed off. Small organic particles (proteins) often have a positive and a negative charged end which makes attachment to the water surface most effective. This removal mechanism is particularly effective in salt water applications as formation of fine bubbles is much easier than in fresh water applications. Foam Fractionators (also known as Protein Skimmers) use this technology and are often incorporated into Recirculating Aquaculture Systems (RAS) for removal of particles that are not settleable or filterable.

Filtration systems employ barriers which intercept particles while allowing the flow to pass. Two types of measurement are used in gauging filter effectiveness: absolute and nominal. Where precise hole sizes are used, as in screening systems, the filter will retain all particles larger than the hole size which is referred to as the “absolute” filter size. In practice, as particles accumulate on the screen surface, a significant percentage of particles smaller than the absolute hole size will also accumulate on the screen. This is usually measured in terms of percent captured and is called the “nominal” filter size. Because hole sizes cannot be controlled in media filters due to uncertainty in packing arrangements, media filtration system sizes are all considered to be nominal.

Media filtration systems rely primarily on the interception of particles larger than the interstitial spaces between the grains of the media. However, passage of water through the media must follow a convoluted path which improves the capture efficiency. Media with special properties can also be used which will help capture fine or dissolved materials. Deep media beds and/or fine grain media will improve particle capture, but also require increased operating pressure. In aquaculture, pressurized media filters are primarily used for influent treatment, where removal of inorganic particles is the main goal, or for effluents from low intensity culture systems such as aquariums or laboratories. Effluents with heavy organic loads, which are common in commercial and public finfish culture systems, are unsuitable for media filtration as they will cause the filters to foul rapidly.

In screen filtration systems, opening sizes can vary from several millimeters when coarse wire screens are used to less than 0.001 micron in the membranes used in reverse osmosis and nano-filtration systems. Fine filtration systems typically require much larger filter screens and/or higher pressures than coarse filters. In aquaculture systems, microscreen drum filters, with screen openings ranging from 9 to 250 micron, are the most commonly used screen filtration system. Drum filters are integral components in Recirculating Aquaculture Systems, because they provide fully-automatic, uninterrupted filtration with low operating head and backwash water requirements. They are also commonly used in both influent and effluent treatment systems for aquaculture facilities.

Whether you require an influent, effluent, or recirculation treatment solution, PR Aqua can assist you in selection of a solids removal technology for your facility. PR Aqua’s design experts can help you integrate solids removal into your existing process or can package solids removal with other processes, providing a custom solution to meet your broader treatment objectives.