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A biological combined treatment for wastewater in the freezing industry

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One of the most important challenges of the industries, in general, is to be able to meet the requirements that the administration demands for the wastewater discharges as a product of its industrial activity.

This was one of the issues raised by Congelados Navarra, one of the largest producers of frozen vegetables and a reference in the sector in Europe.

A company that has maintained an important sustained growth during the last years, with a consolidated national and international expansion plan, which has led them to obtain a growing need of extension and improvement in the wastewater treatment facilities of their different production centers.

On the other hand, Congelados Navarra has planned, within its strategic objective of continuous improvement and optimization of resources, to carry out a program of evaluation of the manufacturing processes in such a way that water consumption per unit produced has been reduced.

One of the most important challenges, in terms of the environment and water treatment, that the company faced was that of its main plant in Fustiñana (Navarra).

The frozen food company needed its facilities to comfortably comply with the new requirements requested by the administration and, also, with the lowest possible energy consumption, sludge generation, and environmental impact.

The technological solution that has been installed is a combined process of Anaerobic Treatment RANC, followed by the Aerobic process called Sigma FBR (Flotation Biological Reactor).

Types of biological wastewater treatment for Congelados Navarra

Next, we explain in detail the processes and treatment stages applied in the main plant of Congelados Navarro in Fustiñana.

The processes and their stages are:

1. Water screening

Initially, the waters pass through a rotary screen with capacity for the entire flow with a 1 mm filter.

2. Homogenization of waters

The homogenization tank, mainly intended to stabilize flow and the polluting load, has a volume of 1,000 m3.

The equalizer has a system of AQJETS for agitation and aeration so that sediments and degradation of organic material are avoided in conditions of oxygen starvation, generating unwanted odours. There will also be a cover for the equalization tank in order to control possible odours and collect them to lead them to a deodorization system.

The pH control is carried out in the homogenization tank.

3. RANC reactor.

To minimize the operating costs of energy consumption and the generation of sludge, we chose an anaerobic digestion system with a capacity of 1.800 m3.

Inside, organic compounds (COD and BOD) are degraded under anaerobic conditions, mainly transformed into treated water and biogas.

The type of reactor will be “ascending flow by contact”, which has similar operating conditions to the classic UASB, but, in this case, the three-phase clarifiers are excluded, and the waters are clarified at the outlet of the digester using a DAF type float.

The sludge from the anaerobic reactor must be separated before the next stage. For this phase, we use a BIODAF floatation clarifier type.

The most significant advantages of the BIODAF clarifier are:

  • Great reliability against spongy muds with low sedimentation speed.

  • The high concentration of extracted mud, between 3-5%, causing an improvement of the systems’ hydraulic conditions, a decrease in returned of water to homogenization, and the centrifuge will manage a smaller volume of mud during treatment.

The difference between RANC technology for anaerobic digestion, which also requires an external DAF clarifier, and the classic UASB with internal clarification, is the existence of an intermediate DAF acting as a “by-pass”. This DAF will enable part of the flow coming from situations of system maintenance, the system resets or peak flows to go into the anaerobic reactor; acting in these cases as a physical-chemical process to support the final biological reactor.

4. FBR Biological oxidation reactor

The final plant has one low-medium load active sludge reactor with a volume of 3,750 m3 separated into two tanks.

The aeration system is a submerged ejector type with pressurization, which guarantees maximum transfer with minimum installation maintenance.

5. BIODAF Final clarification

The particular conditions of the biological sludge generated in industries where conditions of adequate nutrient balance do not exist, give rise to the formation of very voluminous and sparkling sludge.

The sedimentation speed determines the size of the final decanter, which is usually calculated for biological sludge, in this situation, where the sludges’ speed drops to 0.4-0.5 m/h, a large decanter will be the right choice.

Besides, the high volume occupied by the sludge makes it necessary to recirculate a massive flow of sludge to prevent this from escaping from the decanter, which complicates, as the client has already experienced, the treatment of mud and the hydraulic return to homogenization.
We suggested using a DAF flotation system as the final clarifier instead of the traditional decanter. The use of DAF as a solution for these types of situations in industrial plants is increasing, and in our case, we have carried out many of these types of installations.
The most significant advantages are:
• Great reliability against spongy muds (bulking) or with low sedimentation speed.
• Ability to work up to 8 gr/L of high solids loads in the reactor, which allows the sludge to age more and improve water treatment conditions.

6. Sludge treatment

The plant has a sludge accumulation tank from which it is treated in a centrifugal decanter type dehydration system.
Because these are muds with an organic origin that are mostly digested, they are potentially usable as fertilizers in cereal crops, among others.

Combining technology for better results

The Congelados de Navarra facilities are a clear example of the combination of technologies to achieve the best results while respecting environmental regulations.

The plant treats up to 2400 m3/day of wastewater with maximums of 18,000mg/l of COD, reaching final values below 100mg/l.

It is also a model of how a SIGMADAF equipment can be used as clarifiers for biological processes, both aerobic and anaerobic, providing reliability of operation and results that will not be achieved by other solutions.