Treatment of seafood processing wastewater

Aquatic wastewater treatment is a mandatory requirement for the seafood processing industry. The seafood processing industry is currently one of the important economic sectors, with a large scale of commodity production, at the forefront of international economic integration. However, the seafood processing industry is also one of the industries that causes serious pollution to the environment.

Video of Aquatic wastewater treatment technology designed by Nam Viet

Sources of wastewater from seafood processing

• The process of washing raw materials, machinery, equipment, and factories
• Fish waste, leftover food scraps…
• Washing and cleaning process of employees

The amount of wastewater in the seafood processing industry accounts for 85-90% from the production stage, mainly the stages of handling raw materials, processing, finishing products, cleaning factories and tools and equipment.

The rest is domestic wastewater from employees, cleaning of the dining area, kitchen… in the factory.

Some reference data for wastewater from the seafood processing industry

• Catfish: 5 – 7m3/ton of product.
• Frozen shrimp: 4 – 6m3/ton of product.
• Surimi (imitation crab products): 20 – 25m3/ton of product.
• Mixed frozen seafood: 4 -6m3/ton of product.

Wastewater from this industry contains mostly organic waste of animal origin and is mainly composed of proteins and fats. Of these two components, fat is difficult to decompose by microorganisms.

Organic substances contained in seafood processing wastewater are mainly easily decomposed.

Wastewater contains substances such as carbohydrates, proteins, fats, etc. when discharged into the water source, it will reduce the concentration of dissolved oxygen in the water because microorganisms use dissolved oxygen to decompose organic substances.

Dissolved oxygen concentrations below 50% saturation can potentially affect the development of shrimp and fish.

Reduced dissolved oxygen not only causes degradation of aquatic resources but also reduces the self-cleaning ability of water sources, leading to reduced water quality for domestic and industrial use.

Effects on the environment of the seafood processing industry

• Air pollution:
  • Bad smell due to storage of production waste
  • Emissions from backup generators..
• Solid waste generated mainly from the processing process includes shrimp heads, clam shells, squid skin/shell, squid and fish organs,….
• Wastewater produced in seafood processing accounts for 85-90% of the total amount of wastewater, mainly from the following stages: washing in raw material handling, processing, product finishing, factory cleaning and utensils. tools, equipment, and domestic wastewater.

Composition and properties of seafood processing wastewater

Aquatic wastewater is one of the very typical types of wastewater, capable of causing high environmental pollution with its content of organic matter, suspended sediment, N, and especially P and disease-causing organisms.

• Organic compounds account for 70–80% including proteins, amino acids, fats and their derivatives in the product. Most organic substances are difficult to decompose.
• Inorganic matter accounts for 20–30% including sand, soil, salt, urea,  additives….
• N and P: Seafood processing wastewater often contains very high levels of N and P, especially Phosphorus, which is not only present in the ingredients of the product but also arises from the process of adding additives. processing
• COD and BOD in aquatic wastewater are very high
• Disease-causing microorganisms: contains many types of bacteria, viruses and helminth larvae that cause disease.

Composition and properties of some types of aquatic wastewater.

Source: General Department of Environment 2009

Aquaculture Wastewater Standards after treatment meets QCVN 11-MT:2015/BTNMT standards

Some wastewater criteria as a basis for designing treatment systems

(Source: QCVN11-MT:2015/BTNMT).

Technology diagram of seafood processing wastewater treatment

• The main ingredients are biodegradable organic compounds, nitrogen compounds, and high phosphorus. Therefore, the biological method applied is very effective
• Choosing the method depends on many factors: output standards, composition, wastewater flow and treatment cost.

Collection – Preliminary fat separation

Collecting wastewater from the factory to a centralized treatment area, combined with preliminary grease separation through grease separators helps remove a large amount of crude grease, limits pump clogging as well as removes a large amount of residue and soil. sand.

Wastewater will be pumped into the Equalization Tank by a submersible pump.

Conditioning Tank:

In the wastewater treatment system, the equalization tank is built to regulate the flow, concentration of pollutants and neutralize pH (when necessary).

From there, overcome problems caused by fluctuations in flow and pollutant concentration while improving the performance of subsequent processes because:

Substances affecting the treatment process can be diluted, pH can be neutralized and stabilized ⟹ the treatment efficiency of the biological treatment process is enhanced due to no or minimal “shock” ” weight;

The quality of treated wastewater is improved because the waste load on the structures is stable.

Construction area is saved because the structures behind the equalization tank are designed according to the average hourly wastewater flow.

The larger the water storage capacity, the higher the safety in many aspects.

To avoid sedimentation and anaerobic decomposition causing odors, the conditioning tank is continuously aerated or stirred.

Wastewater will be pumped by submersible pumps arranged in sufficient quantity to alternately pump into the super-speed reaction system before entering the DAF tank.

DAF Superfast Flotation

Wastewater after being pumped into the Physicochemical reaction system to mix the water with PAC and Polymer chemicals creates better reaction conditions for the DAF super shallow flotation system.

A Super Shallow Flotation Tank (DAF) is a device used to separate and remove dissolved solids (TDS) from liquids based on changes in solubility under different pressures.

The air is dissolved under pressure in a clean liquid and pumped directly into the Flotation tank.

Once in the tank, air pressure is created and combined with the liquid, which becomes supersaturated with Micro-sized air bubbles.

Tiny air bubbles produce a specific gravitational force that adheres to solid particles suspended in water and lifts the suspended particles to the surface of the liquid, forming a layer of floating sludge that is removed by the filter. rake the surface mud board.

Heavy solids settle to the bottom of the lake and are also raked and sucked out by a sludge pump to be taken to the sludge treatment area for treatment.

Wastewater will automatically flow to the Anoxic Tank for further treatment by biological process.

Anoxic Tank

This biological tank is responsible for denitrification. Bacteria present in wastewater exist in suspended form due to the action of the mixing motor

Denitrification process:

• Occurring in the second step following nitrification, the reduction of nitrate-nitrogen to nitrogen gas, nitrous oxide (N₂O) or nitrite oxide (NO) is carried out. in anoxic environments and requires an electron donor, either organic or inorganic.

There are two possible denitrification pathways that can occur in a biological station:

• Anabolism: The anabolic pathway involves reducing nitrate to ammonia for use in cellular synthesis. It occurs when ammonia is unavailable, independent of oxygen inhibition.
• Catabolism (or denitrification): Catabolic denitrification involves the reduction of nitrate to nitrite oxide, nitrous oxide and nitrogen:

NO₃^–   -> NO₂^–   -> NO(g)  -> N₂O (g)  -> N₂(g)

Some species of denitrifying bacteria are known as: Bacillus, Pseudomonas, Methanomonas, Paracoccus, Spirillum, and Thiobacillus, Achromobacterium, Denitrobacillus, Micrococus, Xanthomonas.

Most denitrifying bacteria are heterotrophic, meaning they obtain carbon for cellular synthesis from organic compounds.

Besides, there are still some autotrophic species, which receive carbon for cell synthesis from inorganic compounds.

For example, the species Thiobacillus denitrificans oxidizes element S to create energy and receives carbon source for cell synthesis from water-soluble CO₂ or HCO_{3</sub >^–.}

Biochemical equation of biological denitrification process:

Depending on the carbon-containing wastewater and nitrogen source used.

• Energy equation using methanol as electron acceptor:

6 NO₃^–  +  5 CH₃OH  -> 5 CO₂   +  3 N₂  +  7 H₂O  +  6 OH^{-</sup >}

• The entire reaction includes biomass synthesis:

NO₃^–  +  1.08 CH₃OH   + 0.24 H₂CO_{3< /sub>   -> 0.056 C5H7O2N  +  0.47 N2  + 1.68 H2O  +   HCO3^–}

O₂  +  0.93 CH₃OH   + 0.056 NO₃^–   -> 0.056 C₅H₇O₂N  +  0.47 N₂  + 1.04 H₂O  + 0.59 H₂CO₃  +  0.56 HCO₃^–

• Energy equation using methanol, ammonia-N as electron acceptor:

NO₃^–  +  2.5 CH₃OH   + 0.5 NH₄^{+</ sup>  +  0.5 H₂CO₃   -> 0.5 C₅H₇O₂N  + 0.5 N_{2</sub >  +4.5 H2O  +  0.5 HCO3–}}

• Energy equation using methane as electron acceptor:

5 CH₄  +  8NO₃^–   -> 4 N₂  +  5 CO₂  +  6 H₂O + 8 OH^–

• The entire reaction including biomass synthesis uses wastewater as carbon source, ammonia-N, as electron acceptor:

NO₃^– + 0.345 C₁₀H₁₉O₃N + H< sup>+ +  0.267 NH₄⁺  +  0.267 HCO₃^–   -> 0.612 C₅H₇O₂N + 0.5 N₂  +2.3 H ₂O  +  0.655 CO₂

The biochemical equation using methanol as the carbon source converting nitrate to nitrogen gas is significant in  design: Reduced oxygen demand 2.86 g/g reduced nitrate.

The alkalinity produced is 3.57gCaCO₃/g reduced nitrate if nitrate is the nitrogen source for cellular synthesis. If ammonia-N is available, the alkalinity produced is lower than 2.9-3g CaCO₃/g reduced nitrate.

Aerotank:

Converts organic compounds into methane and other organic products.

Aerobic biological treatment tanks have a continuous operating mode, treating organic impurities in wastewater with aerobic microorganisms that adhere to fixed substrates inside the tank.

These microorganisms will decompose organic substances into the final products CO₂ and H₂O.

Air here is supplied by air blowers that operate 24/7.

After water leaves this unit, the COD and BOD content is reduced by 80-95%, and the amount of sludge produced is not as much as in the microbiological treatment process with suspended activated sludge (Aerotank tank).

The biological treatment process taking place in the Aerotank is described by the following reaction equation:

• Oxidation of organic matter:

C₅H₇NO₂ + O₂ + microorganisms” CO_{2</sub > + H2O + new cells + energy}

Where C₅H₇NO₂ represents organic compounds present in wastewater.

• Nitrification process:

The Nitrification reaction is described as follows:

• Conversion of Ammonia into Nitrite under the action of Nitrosomaonas bacteria:

Ammonia Nitrogen + 1.5 O₂ → Nitrite + H₂O + reduced alkalinity

• Conversion of Nitrite into Nitrate under the action of Nitrobacter bacteria:

Nitrite + 0.5 O₂ → Nitrate

The nitrification reaction is described by the following general equation:

Ammonia Nitrogen + 2 O₂ → Nitrate + H₂O + Reduce alkalinity

• Nitrate reduction process:

During the biological denitrification process, Nitrate is converted into free Nitrogen gas.

Nitrogen gas produced is released into the air.

In contrast to nitrification, biological denitrification takes place in an anaerobic environment (without oxygen) and uses organic compounds present in wastewater as a carbon source. The Nitrate reaction is described by the following equation:

Nitrate nitrogen + organic carbon → Nitrogen gas + Increased alkalinity

In addition, with the alternating use of aerobic, anoxic and anaerobic processes, phosphorus reduction in wastewater also takes place using both aerobic and anaerobic biological methods.

• Biological phosphorus reduction process:

Phosphorus exists in wastewater in the form of orthophosphate, polyphosphate and organic phosphorus. During biological treatment, phosphorus in wastewater is separated through the formation of tissue by microbial cells during the process of organic matter reduction.

Biological settling tank:

Sediment of microbial sludge flocs from biological processes and separation of these flocs from wastewater.

Wastewater from the aerobic biological tank is led into the central pipe to distribute evenly over the entire horizontal area at the bottom of the tank.

The central pipe is designed so that the water leaves the pipe and goes up at the slowest speed (in a static state), then the formed sludge flocs have a density large enough to overcome the upward speed of the wastewater flow. settle to the bottom of the settling tank.

Excess sludge settles at the bottom of the settling tank.

At the center of the bottom of the tank, there are 2 submersible sludge pumps. The sludge pipeline is divided into 2 directions: 1 is circulation to the Anoxic and Aerotank tanks, 2 is the sludge discharge line through the sludge compression tank and then pressed through the screw sludge press. Multi-discs help sludge drier and recover a large amount of water trapped in wastewater sludge. After pressing, dry sludge will be periodically transferred for treatment by a waste treatment unit.

Flocculation tank:

Wastewater from the biological sedimentation tank continues to flow to the flocculation tank and at the same time, coagulation chemicals (PAC) are also added to the tank.

At the tank, the stirrer motor rotates at a speed of 70 – 150 rpm to create a turbulent flow that completely mixes the chemicals with the wastewater stream so that the reaction process occurs faster.

The amount of PAC chemicals added to the tank will be calculated through the Jartest experiment to select the most suitable chemical concentration for the specific characteristics of each different seafood processing company.

After that, the wastewater will continue to flow through the flocculation tank (At the same time, flocculation aid chemicals are also added to the tank).

Floating tank:

Wastewater from the flocculation tank will be overflowed into the flocculation tank to use flocculation aid chemicals (Polymer) to increase the adhesion of the floc.

Use a stirring paddle at high speed to mix the flocculating chemical with the wastewater stream.

Slow stirring motor of 10 – 50v/min helps mix chemicals with wastewater completely but does not break the adhesion between flocs.

Thanks to the flocculation aid, the formed flocs stick together to form larger flocs with a density many times greater than the density of water, so it is easy to settle to the bottom of the tank when settling and separate from the wastewater stream. .

Wastewater from the flocculation tank continues flows automatically through the physical and chemical settling tank.

Physical and chemical settling tank:

The flocculation process will generate and continuously increase the amount of sludge. Therefore, physicochemical settling tanks are designed to collect this amount of sludge.

The settling tank is designed to create a static environment for the sludge to settle to the bottom of the tank and be collected in the center thanks to the sludge collection system installed at the bottom of the tank.

The sludge after settling is sent to the sludge tank.

The clear water after settling is recovered by a serrated water collection trough system placed on the tank surface and continues to be led to the disinfection tank.

The physicochemical sedimentation sludge will be periodically pumped to the sludge compression tank and then pressed through a multi-disc screw sludge dewatering machine.

This sludge contains a high concentration of solids (2-3%); Therefore, it can be pumped directly to the sludge tank without needing to be compressed and then pressed.

Sludge after pressing is also periodically collected and processed by a waste treatment unit with the function of collection.

Disinfection tank

Wastewater after physicochemical treatment still contains about 10⁵ – 10⁶ bacteria in 100ml, most of these bacteria that exist in wastewater are not bacteria. cause disease, but some bacterial species that can cause disease cannot be excluded.

When Javel/Chlorine is added to water, the Javel/Chlorine chemical, which has strong oxidizing properties, will diffuse through the microbial cell envelope and cause reactions with enzymes inside the microbial cells, destroying the metabolic process. substances that lead to the destruction of microorganisms.

Sludge compression tank

Task: Excess sludge generated from the biological settling tank and sludge from the physical and chemical settling tank will be sent to the sludge compression tank

To separate the water phase from the solid phase using the gravity sedimentation method, combined with a sludge compactor installed at the bottom of the tank to support better sludge compression, optimal for squeezing the sludge for drier sludge.

It will then be pumped into the sludge press to process dry sludge and then transferred to the unit with the function of post-pressing sludge treatment. 

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