Combined biological treatment for wastewater in frozen food production

Scenario

Congelados de Navarra is one of the largest producers of frozen vegetables in Europe and a reference company in the sector. With an important sustained growth in recent years and a consolidated national and international expansion plan. As a result of this, it has an increasing need to expand and improve its wastewater treatment facilities for its various production centers.

The frozen food company needed its facilities to be fully compliant with the new requirements requested by the administration and, in addition, with the lowest possible energy consumption, sludge generation and environmental impact.

The technological solution that has been installed is a combined process of anaerobic RANC treatment, followed by the aerobic process called SIGMA FBR (Flotation Biological Reactor).

Description of the plant

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

Processes and stages of the process

Water screening

Initially, the water passes through a rotary screen with capacity for the entire flow with a 1mm filtration lumen.

Water homogenization

The homogenization tank, mainly intended to neutralize the flow rate and pollutant load values, has a volume of 1,000 ^m3.

The equalizer has an AQJETS system for agitation and aeration to avoid sedimentation and degradation of organic matter in conditions of lack of oxygen, generating undesirable odors.

In addition, a cover will be provided for the equalization tank to control possible odors and collect them to be conveyed to a deodorization system.

pH control is performed in the homogenization tank.

RANC Reactor

To minimize operating costs derived from electricity consumption and sludge generation, an anaerobic digestion system with a capacity of 1,800 ^m3 is available.

In this process, organic compounds (COD and BOD) are degraded under anaerobic conditions, transforming them mainly into treated water and biogas.

The reactor will be of the contact upflow type, which has operating conditions similar to the classic UASB, but in this case the three-phase clarifiers are eliminated and the water is clarified at the outlet of the digester by means of a DAF type float.

The excess anaerobic sludge from the reactor must be separated before the next stage. A BIODAF type flotation clarifier is available for this purpose.

Most significant advantages of the BIODAF clarifier

• Greater reliability against spongy sludge with low settling velocity.
• High concentration of the extracted sludge, between 3-5%, so that the hydraulic conditions of the system as a whole improve, the return of water to homogenization decreases, and a smaller volume of sludge is managed in the centrifuge treatment.

The use of the RANC anaerobic digestion technology, which requires an external DAF clarifier, as opposed to the classic UASB with internal clarification, is that the existence of this intermediate DAF allows BY-PASSAR in case of need all or part of the flow to the anaerobic reactor, for maintenance, reactivation or flow peaks; acting in this case as a physicochemical process to support the final biological reactor.

FBR Biological oxidation reactor

The final plant has an activated sludge reactor of low to medium load with a volume of 3,750 m3. Separated into two tanks.

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

BIODAF Final clarification

The particular conditions of biological sludge generated in industries where there are no conditions of adequate nutrient balance, lead to the formation of very bulky and spongy sludge.

If the sedimentation velocity, which determines the size of the final settling tank, normally calculated for biological sludge, for example from a municipal plant, is between 0.7-0.8 m/h, in the case of these sludges the velocity drops to 0.4-0.5 m/h, with the consequent need for large settling tanks.

In addition, the large volume occupied by the sludge makes it necessary to recirculate a large flow of sludge to prevent it from escaping from the decanter, which complicates, as you have already experienced, sludge treatment and hydraulic return to homogenization.

We have proposed to use a DAF flotation system as a final clarifier instead of the traditional decanter. This technical solution is being increasingly used in industrial plants, and in our case we have realized many installations of this type.

Most significant advantages

• Greater reliability against bulking sludge or sludge with low settling velocity.
• Ability to work with very high solid loads in the reactor, up to 8 g/l, which allows for greater sludge age and improved water treatment conditions.
• High concentration of the extracted sludge, between 3-5%, so that the hydraulic conditions of the system as a whole improve, the return of water to homogenization decreases, and a smaller volume of sludge is managed in the centrifuge treatment.

Sludge treatment

The plant has a sludge accumulation tank from which the sludge is treated in a centrifugal decanter dewatering system.
Because the sludge is of organic origin, mostly digested, it can potentially be used as fertilizer for cereal and similar crops.

Performance and results

The Congelados de Navarra facility is a clear example of the combination of technologies to achieve the best results while taking environmental constraints into account.

The plant treats up to 2,400 ^m3/day of wastewater with maximum COD levels of 18,000 mg/l, achieving final values below 100 mg/l and all this with very low operating costs.

It is also a model for the use of SIGMADAF equipment as clarifiers for biological processes, both aerobic and anaerobic, giving the system as a whole a reliability of operation and results that cannot be achieved with other solutions.