Is it difficult to treat aquaculture wastewater? Choosing the right process is the key

Aquaculture wastewater is rich in organic matter, pathogens and nutrients (such as nitrogen and phosphorus). Direct discharge will lead to eutrophication of water bodies, causing problems such as water hypoxia and death of aquatic organisms. In addition, pathogens and viruses in wastewater will spread with the water flow, which may lead to the outbreak of epidemics.

Shanqing Environment has confirmed in previous engineering practices that biochemical treatment of fecal wastewater from large and medium-sized pig farms after solid-liquid separation is feasible and efficient, but for smaller scale farms, a combination of hydrolysis and acidification and aerobic treatment is more appropriate.

1. Overview of aquaculture wastewater

The types of wastewater discharged from farms include: fecal wastewater, industrial wastewater, and domestic wastewater.

Estimation of wastewater from livestock farms:

1. Water volume: 18-37 liters/(day/head) for domestic pigs, 56-256 liters/(day/head) for cattle and sheep;

2. Suspended solids in wastewater: 15-30g/L for domestic pigs, 30-50g/L for cattle and sheep;

3. COD in wastewater: 15-30g/L for domestic pigs, 35-60g/L for cattle and sheep,

4. The BOD5 in wastewater is 6.25-12.5 g/L for domestic pigs and 4-6.7 g/L for cattle and sheep.

In addition, it also contains a large amount of nitrogen, phosphorus, potassium, etc.

1.1  Main features:

Aquaculture wastewater has the typical "three high" characteristics, namely: high COD (3000-12000mg/L), high ammonia nitrogen (800-2200mg/L), and high SS (exceeding the standard by dozens of times, dark color, and containing a large number of bacteria, high ammonia nitrogen and organic phosphorus content).

1. 2 Advantages:

It has good biodegradability, concentrated flushing and discharge time, and large impact load.

According to the characteristics of water quality, the coagulation and sedimentation process is used to first remove suspended matter and color; biochemical treatment is used to remove organic matter, ammonia nitrogen and organic phosphorus.

2. Processing technology

2.1  Common processing technology:

Engineering practice has confirmed that biochemical treatment of fecal wastewater from large and medium-sized pig farms after solid-liquid separation is feasible and efficient. For small-scale farms, a method combining hydrolysis and acidification with aerobic treatment is more appropriate.

The use of anaerobic digestion can, on the one hand, reduce energy consumption and lower operating costs; on the other hand, it can also recover and utilize methane gas, thereby achieving the purpose of waste utilization.

Biochemical treatment generally uses biofilm method to treat aquaculture wastewater. It generally includes fixed bed biofilm method (contact oxidation), MBR suspended carrier biofilm method, fluidized bed, aerated biological filter, biological rotary disc, membrane bioreactor, etc.

2.2  The characteristics are:

Bio-rotating disc: generally used in large-scale above-ground treatment facilities, it occupies a large area, has poor efficiency, and is prone to secondary pollution;

Membrane bioreactor membrane: difficult to replace, short life, and troublesome to clean;

Aerated biological filter: Although it is more suitable for the treatment of aquaculture wastewater and has many successful experiences, it is not recommended to use it here because of its high noise, the need for a special fan, and the small amount of water in this project;

Contact oxidation combined with high-efficiency new biological filler: It has the characteristics of high load, easy biofilm formation, and mature technology, so contact oxidation technology is used as the main treatment unit in this plan.

To sum up, based on the characteristics of pig farm wastewater, we choose "three-stage sedimentation + flotation machine pretreatment + Bardenpho + filtration + disinfection" as the treatment process for this type of wastewater.

2.3  Introduction to sewage treatment process

The biogas slurry first enters the tertiary sedimentation tank to initially precipitate the heavier suspended matter to reduce the load for subsequent treatment. The supernatant after sedimentation enters the regulating tank. The main function of the regulating tank is to balance the water quality and quantity so that the system can operate under a relatively stable condition.

The water in the equalization tank is pumped into the mixing area of the flotation after the sewage is reacted with the dosing agent, and then mixed with the released dissolved air water, so that the floccules adhere to the tiny bubbles and then enter the flotation area.

The floccules float to the water surface under the action of air buoyancy to form scum. When the scum on the water surface accumulates to a certain thickness, it is scraped into the air flotation machine sludge tank by the scraper and then discharged into the sludge tank. Part of the clean water in the lower layer flows back to be used for dissolved air, and part of it enters the clean water tank, and then enters the biochemical system under the action of the pump.

The biochemical sewage treatment system consists of (hydrolysis acidification tank, primary contact oxidation tank, secondary contact oxidation tank, sedimentation tank). The sewage is acidified in the hydrolysis acidification tank. Through hydrolysis and under the action of acid-producing bacteria, the large-molecule, difficult-to-degrade organic matter in the wastewater is decomposed into small-molecule organic matter, part of the COD and soluble organic acids are removed, and the water quality and water volume of the wastewater are adjusted to ensure the stability of the subsequent treatment load.

It flows into the biological contact oxidation tank through the hydrolysis acidification tank for biochemical reaction. Under the condition of sufficient oxygen supply, the aerobic microorganisms in the biological contact oxidation tank use the organic matter in the sewage as nutrition and carry out their own metabolic activities by decomposing and absorbing the organic matter, thereby achieving the effect of removing the organic matter in the sewage.

In order to ensure the aerobic treatment effect, diaphragm aerators and elastic three-dimensional fillers are installed in the system. The equipment converts ammonia nitrogen and other components into nitrogen and ammonia through aeration. The equipment adds elastic fillers to improve the aerobic effect and increase the area of the biofilm, increase the biomass in the aeration tank, and improve the organic matter removal rate. It has the characteristics of stable treatment effect, high volumetric load, low sludge yield, and low moisture content of residual sludge.

A certain concentration of activated sludge must be maintained in the biological contact oxidation pond, and the sludge source must be returned from the sedimentation tank. This ensures the stable operation of the entire system, maintains a high organic matter removal rate, and effectively prevents sludge swelling.

The mud-water mixture after aerobic treatment enters the secondary sedimentation tank, where the mud-water mixture is separated from the water. The precipitated sludge is returned to the hydrolysis acidification tank for denitrification reaction to remove ammonia nitrogen in the sewage. The remaining sludge is discharged into the sludge pool. The treated sludge can be entrusted for external transportation and disposal.

Through the above process, the harm of aquaculture wastewater to the environment can be effectively reduced to protect the ecological environment and human health. Shanqing Environment can provide technical solutions, equipment supply and installation and commissioning services related to aquaculture wastewater treatment. If you have any questions about aquaculture wastewater treatment, please feel free to contact us for consultation.