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Water treatment in the circular economy

  • Water treatment in the circular economy
  • For TecnoConverting, closing the water cycle in large urban and industrial centres through the reclamation and reuse of waste water goes beyond responsible stewardship of the environment and natural resources, becoming an unquestionable social responsibility, and a critical factor for economic growth.
Content by:Francesc Paradell

About the entity

We are specialists in decanting and lamellae, we carry out engineering, custom design, construction and assembly of the best technical solution for our customers.


The urban ecosystem of our towns and cities is tightly linked to nearby water sources that provide drinking water to the population, sustain economic activity, and receive waste water. In addition, after the industrial revolution, many settlements originated around large industries, while industrial activities moved closer to urban centres, becoming part of the urban fabric and important stakeholders that use resources and demand services.

Water has always been a key resource for industry, linked to the development of a set of services to use it that currently are conceived as a closed cycle (withdrawal, purification, distribution, sanitation and reclamation-reuse), imitating nature, which we should not stray from as we strive for continuous development and growth.

The design of TecnoConverting allows working with one clarifying stage, obtaining the effluent quality required for reuse

In this context, TecnoConverting's lamellar technology has enabled the optimisation of the treatment and reuse of water from the gas scrubbing system of a secondary aluminium smelter plant in the province of Barcelona.

The project implemented by TecnoConverting entailed the complete refurbishing of the existing water treatment facilities to expand the gas scrubbing capacity. The capacity of the new plant was set to be 30 m3/h, exceeding the 12 m3/h of the old facilities.

The water to be treated comes from the gas scrubber in the smelting furnace area, it has a high metallic solid content, particularly aluminium, and a basic component due to the use of salts as adjuvant substances in metal smelting.

TecnoConverting has developed the project following an EPC model, including initial jar tests to select chemical reagents.

Given the basic pH of the water and the high aluminium concentration, the plant designed by TecnoConverting includes an initial phase that neutralises water by adding sulphuric acid, which also acts as a coagulant, automatically rendering insoluble the high metallic compounds content. A new neutralisation tank with a capacity of 4 m3 has been built for this, made of stainless steel and with a square base, that receives the dirty water from the gas scrubber and has a retention time of 8 minutes. Water in the tank is stirred with a vertical 100 rpm stirrer, which was reused from the old facilities.

The return flows from the new plant have also been connected to the neutralisation tank, to prevent water overflows in the tanks if ever the treatment capacity of the facilities is exceeded. The water from emptying the flocculation tanks has been connected to the neutralisation tank too. This last point is important because the plant, according to the production requirements of the industry, works 8-10 hours per day, so the flocculation tanks are emptied daily when the plant stops, to prevent water from becoming stagnant and the flotation of solids when the facilities start up the next day.

From the homogenisation tank, water rises using a centrifugal pump with a variable-frequency drive, to two stirred flocculation tanks in series: the first one, with a capacity of 3 m3, was newly built in stainless steel, and the second one, with a capacity of 2 m3, was reused from the old plant. The flocculant is added in the first of the tanks, and the total retention time for flocculation is 10 minutes. Each of the flocculation tanks has a vertical 25-30 rpm stirrer.

Afterwards, flocculated water moves on to a new compact lamellar clarifier, shaped like a vertical truncated cone, with TecnoPlate lamellar technology (flat lamella), designed to treat the entire new flow in the plant, with a Hazen speed of 0.92 m/h and an equivalent lamellar retention time of 2.62 h. Water enters the clarifier through a settling tank to flow then to the lamellar area with no turbulence. To facilitate maintenance tasks, TecnoConverting has placed the flat lamella on three frames with individual guides for each lamella; this allows them to be extracted individually, and special cleaning means are not required. In addition, the clarified effluent from the new tank flows out through side Thompson channels, thus the lamellar area can be accessed without any obstacles. The clarifier includes in the settling area a spillway connected to the homogenisation tank, to prevent it from overflowing if ever the treatment capacity is exceeded.

The old plant had two small lamellar clarifiers in series. TecnoConverting has placed this equipment following the new lamellar clarifier, in parallel, as a second clarifying stage to further refine the effluent. The design of TecnoConverting allows working with a single clarifying stage, obtaining the effluent quality required for reuse, or with two stages, if we want higher quality or we estimate the raw water inflow has an excess load. Moreover, TecnoConverting has planned a connection that bypasses the new clarifier and takes water to the second clarifying stage, which allows the process to continue if maintenance or cleaning work is required in the new clarifier.

The treated water returns to the clean water tank of the scrubber, located under the cleaning tower, to be reused. The quality of the effluent obtained allows full water reuse in a completely closed cycle. Only small potable water inputs are required due to the evaporation that occurs when warm gas comes into contact with the water shower inside the scrubber.

With the facilities planned by TecnoConverting we can close the industrial water cycle and reduce water consumption to a minimum

Concerning removal of sludge, the new clarifiers have been connected to the filter bag system of the old facilities through pipes and valves; this operation is done manually by plant personnel.

With regard to dosing of reagents, TecnoConverting has installed a dosing panel for the sulphuric acid used for the neutralisation and coagulation of raw water; it comprises an electromagnetic dosing pump with a 6-10 l/h flow, regulated through a pH meter in the neutralisation tank. Based on the volume used, we opted for a product storage tank with a capacity of 1 m3, with a spill containment tray, and located in a covered outdoor location. The dosing panel includes an electronic flow sensor and a multi-function overpressure-vacuum valve, as well as a product aspiration tube with a level sensor. All reagent pipes are made of polyvinylidene fluoride (PVDF), and are also protected with transparent PVC tubes.

The chosen flocculant is a concentrated liquid anionic polyelectrolyte that ensures the maximum performance with the minimum dose. A second dosing panel has been installed for it, based on an electromagnetic pump for concentrated product, a stirred 120 litre dilution tank with level gauges, an automatic potable water inflow system, and a dosing 50-300 l/h piston diaphragm pump for diluted flocculant. In this case, the dosing pipes are made of PVC.

The facilities designed by TecnoConverting have been completed with an electrical panel to control and power the equipment, including the new and the old equipment reused from the old plant.

Energy efficiency in the plant has been optimised, favouring gravity-driven water movement and sludge removal

TecnoConverting built all the equipment and pipes, including the lamella, out of AISI 316 stainless steel, considering the high temperature of the water to be treated (50 ºC) and corrosion, except for the reagent dosing pipes which were made out of plastic. Moreover, energy efficiency in the plant has been optimised, favouring gravity-driven water movement and sludge removal.

TecnoConverting has developed the project following an engineering, procurement and commissioning (EPC) model, including initial jar tests to select chemical reagents. The plant was delivered to the client completely finished and in operation, and basic training was provided to plant staff.

With the facilities planned by TecnoConverting we can close the industrial water cycle and reduce water consumption to a minimum. The new facilities, together with the aluminium recycling done at the plant, are clear examples of a 'circular economy', and of sustainable and socially responsible development.

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