In this guide, we will go through the different stages and processes you can deal with in Aquaculture effluent treatment.
Managing aquaculture effluents
When you are considering a Recirculating Aquaculture System (RAS) it is always important to remember that regardless of how much you want to recycle the water stream, there are always effluents to consider specifically, from the mechanical filter backwash and potentially biofilter backwash. All of the unconsumed food along with the fish excrement needs to be dealt with but regularly we forget about this part of the process because it is, in reality, a fairly small component of the overall mass balance. However, it is a critical thing as there are other parameters to consider: Nitrogen, Phosphorous, COD, suspended solids etc.
All parameters need to be evaluated depending on the discharge requirements from the Country, Region and Corporate mandates that need to be accounted for. It is with this in mind that we felt we should put some understanding around the basic processes utilized.
Solids mechanical dewatering
Let’s walk through the solids first as they are technically the easiest part to work with. In general, the percentage of solids that come from the backwash of the mechanical and biological filtration is usually below 0.1% solids. Therefore, we obviously want to try and recover as much water as possible from this backwash while concentrating the solids to the optimal level (depending on storage, logistics and disposal options).
In this Alumichem system, the first stage of sludge thickening is performed in a drum thickener. A drum thickener utilizes gravity and rotation to allow the water to drain through a porous medium. It is also important to note that if you are going to be using a coagulant and flocculant for improved solids-liquid separation and chemical contaminant removal (see Chemical Contaminants) the first injection point would be into the slurry entering the drum thickener. The slurry and chemistry are added to the drum thickener and then carried to the bottom of the rotating drum. The drained water is gravitated through the filter end and the up-concentrated sludge is carried to the sludge discharge. At the sludge discharge from the drum thickener, we now have a dry matter of 5-10%.
From the drum thickener we then move to the next stage of dewatering which is typically a decanter centrifuge. The goal of the decanter centrifuge is to convert the dry matter content from 5-10% up to 30% solids. This is accomplished by centrifugal forces (rotation) that separates components of different densities. Ultimately you drive the solids to the outside due to their heavier weight. With the solids pushing to the outside, you can collect the clean water that is free from solids in the middle (see Water Effluent Polishing). (Here is an animation video of an operating decanter Alfa Laval decanter centrifuge – YouTube) . This concentration effect from the decanter leaves us with a 30% dry matter sludge, which is now like moist soil (see Photo 2).
To further concentrate the solids slurry and bring it up to 90% dry matter then you will need to look at drying. This step can take our dry matter content above the 90% level which will leave us with an easy-to-handle gravel like product. The technology that is employed at this point is a sludge dryer. With drying there is a potential for a high energy demand, so it is important to evaluate what type of system is right for you. There are pure electric dryers, steam dryers, friction dryers and there are dryers which employ the use of a heat pump.
Regardless of your choice the process is relatively simple: the 30% dry matter sludge is spread out to achieve the largest amount of surface area (typically a conveyor belt is used, but there are other systems that employ other methods to achieve the surface area required) and then thermal energy (heat) or dry air is applied to evaporate off the water. Again, depending on the system that you choose that heat can be applied via superheated steam, high temperature air or dry air at a low temperature. At the end of the process, you will typically have an easy to manage product (see Photo 1) that can be disposed of or re-used in a variety of ways. There is secondary benefit to drying as it provides a hygienic product due to the thermal denaturing effect of the heat applied. At the other end of the process, you may have a condensing system which collects the evaporated liquids (see Photo 3).
Water Effluent Polishing
The last stage of the dewatering process, from a solid’s perspective, is to decide what to do with the water effluents from each of the stages. Depending on your discharge requirements and your desire to recycle further you can look at a polishing stage to ensure that your remaining water streams are at the highest quality. This can be achieved as a simple mechanical filter, or you can look to use membrane technology to provide for the highest quality of water to be reused in your system (see Photo’s 2 and 3 for output reference from an operating aquaculture sludge plant).
The one thing that the mechanical drying techniques don’t necessarily deal with is the removal of the potentially high levels of Nitrogen (N), Phosphorus (P) and COD that can be in the effluent due to the unconsumed food and fish excrement. The Phosphorus and the insoluble portion of the COD can be dealt with through the addition of Coagulants and Flocculants.
Coagulants and Flocculants
Coagulants and flocculants are compounds added to the waste stream that promote the “clumping” of fine particles in a floc which then forms into larger particles so that it can be more easily separated from the water via mechanical dewatering. The coagulant is the starting process of the reaction and involves the neutralization of charged particles and if you utilize the right chemistry this will also allow for the precipitation of Phosphorus and COD. Once your particles are neutralized and begin to form a floc, the flocculant then comes into play to agglomerate the particles into a size that will be easy to remove through the mechanical dewatering process. It is important to be aware that the effectiveness of the chemical program (coagulant and flocculant) is very dependent on chemical products chosen, its dose and mass, the pH and initial turbidity of the water being treated, and the properties of the pollutants present.
Heavy metals removal
The other parameter that you need to be aware of when dosing with coagulants is the potential need to remove heavy metals. Heavy metals removal requires very specific chemistry applications and therefore, your decision on the type of coagulant you choose along with dosing levels is critical depending on what your end use or disposal method for your sludge is
Dealing with Nitrogen
Because we are dealing with a wastewater process we have to employ a more mechanical approach to the removal of Nitrogen and essentially we have to force it through the Nitrogen Cycle by employing varying technologies (Nitrogen Removal Basics). The process we are looking to achieve is the conversion of Ammonia/Ammonium to Nitrogen gas by providing enough surface area and oxygen for autotrophic and heterotrophic bacteria to do the work for us.
Nitrogen removal techniques for wastewater treatment
There are multiple technologies that can be employed in this process but the most common with regards to waste water treatment is the Activated Sludge Process (Activated sludge process) where you facilitate the Nitrification/Denitrification process through the use of settling, aeration and recirculation of bacterial cultures.
Some of the other technologies that can achieve the Nitrification/Denitrification process are the Membrane Bio Reactor, Mixed Bed Bio Reactor and the Fixed Bed Bio Reactor (MBR, MBBR and FBBR (Part 1) – Comparison of Wastewater technologies and MBR, MBBR and FBBR (Part 2) – Comparison of wastewater technologies). Each of these systems has their advantages and disadvantages when it comes to capital expenditure, operational knowledge and resistance to upsets so it is very important to understand your operating conditions and the impact on your system if something goes awry.