Congelados de Navarra

Fustiñana (Navarra)

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ApplicationFrozen Vegetables Producer

Anaerobic Contact Reactor (ACR) and Flotation Biological Reactor (FBR)

EquipmentHomogenization Tank 1000 m3
Anaerobic Contact Reactor 1800 m3 DAF-FPBC100
Biological Reactor 3750 m3 DAF-FPHF-Twin
Flow2000 m3/day
PerfomanceDQO influent 18.000 mg/L
DQO effluent 100 mg/L
SST influent 2.000 mg/L
SST effluent 20 mg/L

Client needs

Congelados de Navarra SA is one of the largest producers of frozen vegetables in Europe and a leading company in the sector. They have had significant, sustained growth in recent years and have a consolidated national and international expansion plan for the future. Of course, as a consequence of their continued expansion, they have had to improve wastewater treatment facilities in their production centres.

The company, with its strategic objective of continuous improvement and resource optimisation, undertook an evaluation of the manufacturing processes. As a result, water consumption per unit produced has been reduced. One of the most critical challenges, in terms of the environment and water treatment that they faced was that their main process plant needed new facilities to fulfil the new discharge requirements that were requested by the Spanish administration.

The company came to us to discuss their needs and objectives, with the aim of also achieving the lowest possible energy consumption, while minimising sludge generation and environmental impacts.


​​​​​WWTP description

We believed the most appropriate solution for this project was the combined Anaerobic Ranc Treatment process followed by the Aerobic process called Sigma FBR (Flotation biological reactor).

The processes and their stages are:

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

• Homogenisation of waters. The homogenisation tank, mainly intended to stabilise flow and the polluting load, has a volume of 1,000 m3.

The equaliser 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 equalisation tank in order to control possible odours and collect them to lead them to a deodorisation system.

The pH control is carried out in the homogenisation tank.

• RANC reactor. To minimise 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.

EThe 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.

• High concentration of extracted mud, between 3-5%, causing an improvement of the systems’ hydraulic conditions, a decrease in returned of water to homogenisation, 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, 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.

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 pressurisation, which guarantees maximum transfer with minimum installation maintenance.

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 Frozen industry Case study experienced, the treatment of mud and the hydraulic return to homogenisation.

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 type 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.


  • 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.



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

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