Water treatment

Mbr: a current and future solution

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Halfway between the present and the future, the MBR system is a proven technology for treating paper mill wastewater. A company with 20 years’ experience in treating wastewater using membranes presents the results obtained in Romania with a tailor-made approach.

Wastewater treatment plants can be found in many different industries. Water, which is indispensable for many production cycles, must be treated after use in order to be disposed of. Wastewater management is therefore not simply a question of choosing the type of plant, but requires very careful assessment to identify the best solution in each case. It is a question of balancing aspects such as the provisions of the law and the limits it rightly imposes, the needs of the production cycle and its characteristics, and sustainability, including economic sustainability for the company.

Cannon Artes adopts just such an approach. The company, which is part of the Cannon Group, has been providing wastewater treatment solutions for various industries since 1977: petrochemical, pharmaceutical, food and, of course, paper.

The Romanian case: discharge in a protected area

«Water is a precious resource; it is certainly the most used but also the most wasted on Earth. The mission of Cannon Artes, through the continuous study of technological innovation, is to provide systems and plants for the treatment of wastewater to allow its sustainable use». This was spoken by Fabio Lucariello, area manager and Pulp&Paper specialist of the company, who presented a very interesting application case at the Aticelca 2021 Congress.

We are in Romania, where the MG Tec group, historically engaged in the sector of metal products, insulation and building materials, and aluminium, since 2012 has taken a new entrepreneurial path with a view to diversifying its business, entering the paper sector. In the town of Dej it has built one of three tissue paper mills.

The group turned to Cannon Artes for the construction of a wastewater treatment plant to serve the production facility. The plant needs to treat wastewater to discharge into a surface water body with a very significant organic load, approximately 3,200 kg per day of BOD (Biological Ozygen Demand) – an amount equal to that of a city of over 50 thousand inhabitants. «The characteristics of the effluent are typical of a paper mill» Lucariello explains, «a high organic load, a high concentration of suspended solids and an unbalanced ratio of carbon, nitrogen and phosphorus, so a nutrient input that needs to be corrected during operation and the plant itself».

«The challenge, which we at Cannon Artes accepted, involved assessing two main critical issues: the high customisation required and the high performance to be guaranteed». Reaching the optimum solution has required a close cooperation with the mill’s technical staff from the outset.

With regard to customisation, one of the first requests from the owners was to reduce the overall dimensions. «It was necessary to create a solution that was perfectly suited to the general layout of the plant, inserting the units in the available spaces both outside and inside the building created by the client and in which the more delicate waste treatment units were then installed».

Achieving the set performance, however, had to take into account the location of the plant. «Once treated, the water is discharged into a surface water body in an area considered to be protected, with limits that are therefore more stringent than those of a conventional discharge into a surface water body».

The three treatment steps

The solution proposed by Cannon Artes is based on three treatment steps: pre-treatment, the MBR biological system, which is the heart of the plant, and the sludge line. Everything is managed by an ad hoc automation system.

«Pre-treatment» explains Lucariello, «consists of an equalisation phase, which is necessary to reduce peak flow rates and feed the downstream treatment with an almost constant concentration of pollutants; a screening phase, necessary to remove coarse solids from the process; and a chemical conditioning phase, which is used to remove suspended solids and colloidal solids from the tributary using gravity processes, in this case DAF (Dissolved Air Flotation). For chemical conditioning, specific chemicals have been selected to enhance and optimise the aggregation of colloidal suspended particles and thus their removal».

The core of the system, however, is the MBR system. «The main stage of the process – filtration – takes place in the oxidation tank, where membranes are used to separate the liquid and solid parts. Pumps extract the water by forcing it through the membrane walls and leaving the dirt outside. The clarified water is removed from the plant and discharged, while the sludge and solids remain outside the membrane. This is followed by the backwash phase, which takes place by reversing the flow of water, which crosses the membrane again, but in the opposite direction, allowing the solids to detach from the membrane body».

Cleaning of the membranes, explains the manager, is entrusted to three types of washing: washing with water only, chemical washing – for which the Romanian mill has chosen to use sodium hypochlorite and citric acid – and the insufflation of air under the membranes which, by creating a tangential force on the surface of the membranes, causes the detachment of the bodies of sludge that accumulate on the membrane itself.

Inside the system

Specifically at the Dej plant, the MBR system has been built with two oxidation tanks, each with a volume of 1,600 m3, which treat 50% of the flow; a choice, the latter, which has brought a double advantage, Lucariello continues, “on the one hand the possibility of managing the plant in a more flexible way, and on the other the possibility of partially treating the reflux even in the event of maintenance of one of the two lines. Inside the oxidation tank, fine bubble diffusers are used to promote the passage of air to the biomass, which is composed of the bacteria that create the sludge, and thus cause it to degrade».

The entire process is controlled by a series of instruments set up inside the two tanks to monitor the operating conditions and ensure that they meet the design conditions. In addition, since a large part of the plant is located outside the factory, different environmental conditions have also been taken into account, taking into account seasonal temperature fluctuations and different loading conditions.

The membranes are the other constituent element of the system. The total installed area is 10,800 m2. «The membranes are made of a plastic material – polyvinylidene fluoride (PVDF) – and have a pore size of 0.03 µm. In addition, they have been selected to ensure the lowest possible energy consumption». The manager recalls that the membranes have also been fitted with dedicated instruments to check operating parameters. In particular, the monitoring involves the concentration of suspended solids in the MBR tank and the intramembrane pressure to ensure their integrity over time.

Sludge treatment

The last step in the system studied by Cannon Artes concerns sludge treatment. The process produces two different types of sludge: a chemical-physical sludge from the floater and a biological sludge. The quantity of sludge produced is considerable, around 180 m3 per day. The excess material, which cannot be used within the process, is sent to a stabilisation tank. «A centrifuge was chosen as the machine to dewater the sludge» explains Lucariello. «It is a machine that can reduce the volume of sludge that is sent for disposal by 15 to 20 times. It’s an important number if you take into account that, in terms of running costs, sludge is one of the heaviest items in sewage treatment plants.

The automation system

One of the cornerstones of the entire plant is the automation system tailored to the Dej paper mill. «Firstly, a control and power supply panel was set up in which all the inverters, both those of the blowers and those of the sludge recirculation pumps, were allocated». As explained by Lucariello, the choice has been made to allow the system a certain flexibility in terms of operation and, at the same time, to reduce energy consumption as much as possible. «The system is equipped with an HMI (Human Machine Interface) and has been designed to promote smart management of all the plant parameters». Through the PLC (Programmable Logic Controller) it manages the digital signals coming from about 50 electrical appliances. «From all this data it is possible to extract the operating credits of the main parameters, set them and batch the data for remote management of the system. The main parameters monitored by the PLC are: the flow rates in and out of each section, the operating pH of the dissolved air float, the dissolved oxygen concentration in the oxidation tank, the intramembrane pressure and the sludge concentration in the MBR tank. In other words, the whole set of parameters that governs the processes and that must be easily managed by the plant operators».

Results and assumptions for the future

The results obtained at the MG Tec group’s Romanian plant demonstrate the validity of the Cannon Artes project.

As far as the performance of the system is concerned, the parameters collected show that in the DAF it is possible not only to achieve the performance targets, but even to go beyond them, reducing the concentration of suspended solids in the inlet by about 90%, the COD (Chemical Oxygen Demand) by about 30% and the BOD by about 20%. While the achievable performance in the biological compartment speaks of a further reduction of 95% in total suspended solids, 90% in COD and 95% in BOD.

The project has also shown that «the use of MBR technology allows us to obtain effluent that has better characteristics than conventional aerobic biological plants followed by tertiary treatment – i.e. with disinfection and filtration – and with an effluent quality that is even higher than the limits required for a surface discharge» adds Lucariello.

In addition to plant yields, other advantages of an MBR system can be of particular interest to paper mills, from reduced plant volume to lower energy consumption. «Since there is no gravity separation of solids, i.e. there is no sedimentator or floater, it is possible to work with a higher concentration in the oxidation tank – up to two or two and a half times higher – than in biological activated sludge plants. The consequence is a drastic reduction in the volume of the oxidation phase. Not only that, the compactness of the MBR tank, compared to sedimentation and floatation units, allows an overall reduction in the overall dimensions of the system of around 60%.

Lower energy consumption, on the other hand, is possible thanks to “the selection of membranes with lower energy consumption and the lack of tertiary treatment».

A final achievement of the Romanian project is related to the customisation aspect. «A tailor-made solution was designed that perfectly respected the structure of the plant and all the constraints that the paper mill had set in terms of layout». 

MBR technology is recognised by the European Commission as BAT (Best Available Technology) and is a starting point for achieving other ambitious results in the future, such as a hypothetical ZLD (Zero Liquid Discharge). «These are concepts that would be technically achievable, but are not economically viable at the moment,» recalls Lucariello. «In any case, the MBR solution certainly represents a first step».

 

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