Jose M. Veza

Dead-end microfiltration as advanced treatment for wastewater

Abstract This paper assesses the results obtained from a microfiltration pilot plant operating with effluent water from an activated sludge reclamation plant as compared with those predicted by two models: the “resistances in series” and the “blocking laws” models. The microfiltration unit consisted of hollow-fibre membranes and it worked with direct flow. the characteristics of the feed water varied with time depending on the treatment plant operation and on the characteristics of raw feed water. Therefore, it was impossible to work with a desired composition and the unit worked with a real and variable feed water. Two sorts of experiments were conducted: a series in which flow rate was kept constant (ranging from 6–7 m3/h during 60 min), and a second block, also in 60-min cycles, where neither the flow nor the transmembrane pressure were controlled. Finally, the experimental data were in good agreement with the model.

Desalination in the Canary Islands: an update

Abstract The Canary Islands are subject to a continuous drought with average rainfall of about 300 mm per year. Therefore, water resources are scarce, and several islands depend strongly on sea and brackish water desalination. Over the last 10 years, an enormous development of desalination activities has taken place. This paper presents a review on the current conditions of desalination technologies in the islands. Three consecutive Regional Desalination Schemes have been agreed upon between state and regional authorities since 1988, with some 30 plants built or planned, providing over 316,000 m3/d production capacity in. Current capacity reaches 330,000 m3/d. Both distillation and membrane technologies are used, although the trend over the last years is for seawater reverse osmosis. The islands have become a live laboratory where all new developments are tested. Sizes vary widely from below 100 to 39,000 m3/d, and applications for product water also vary from urban supplies to industry and irrigation purposes. The modes of management have also evolved. Although main funding is from public authorities, private companies now participate actively in plant financing and operation under contract. Finally a brief section of trends on wastewater reuse and on the expansion of desalination to mainland Spain is presented.

Electrodialysis desalination designed for off-grid wind energy

An electrodialysis desalination plant has been set up and tested to treat brackish water while driven from an off grid wind energy system. The tests were carried out in the framework of a wider scope project, located on Gran Canaria Island (Spain). The main goal of this project was to test and identify the most suitable desalination systems for connection to the above-mentioned medium-scale off grid wind farm. After having previously analysed the behaviour of the system on-grid, the following stage was to develop an operational envelope for the electrodialysis reversal (EDR) unit while operating off grid, i.e., only coupled to the wind farm. The unit included power converters for the membrane stacks (DC-drivers) and variable frequency drivers (VFD) for the feed pumps. The tests were carried out to establish the power intervals for the EDR unit depending on the product flow rate specified as well as water quality. Product flow rate between 3 and 8.5 m3/h, power requirements between 4 and 19 kW, while product water conductivity ranged between 200 and 500 μS/cm were recorded. The desalination unit showed good flexibility, adapting smoothly to variations in wind power, even when sudden drops or rises occurred. The control system, slightly modified from a standard design, can cope with such sudden variations. Good agreement between performance predicted with software and the actual operating performance was observed. The presence of harmonics in the electric system due to DC drivers and VFD became harmful for the control and electric system, and care must be taken through appropriate mitigating measures.

Reuse of reverse osmosis membranes in advanced wastewater treatment

Abstract In areas where tap water has a high salt content, wastewater is not appropriate for reuse in agriculture, particularly for sensitive crops. One alternative is reduction, via desalination, of the brackish character to the secondary effluent. A filtration stage is also required before desalination. On the other hand, used reverse osmosis membranes can be recycled and used as filters in the advanced treatment stage in order to reduce suspended matter contained in the secondary effluent—one advantage being the environmental recovery of solid waste. Used membranes can be treated with strong chemical oxidants to peel off the active separation layer in order to transform them into microfiltration or ultrafiltration elements. Preliminary tests have been carried out with 8″ elements, aimed at comparing membrane performance before and after the peeling process. An index denoted as peeling effectiveness (high flux, high salt passage) is used for comparison. It was soon observed that potassium permanganate was more effective than others, together with sodium hydroxide. Doses around 1000 mg/L KMnO 4 provided the best results. It was also concluded that membrane cleaning, done with sodium bisulphite prior to peeling, was better when recirculating the cleaning solution around the membrane rather than soaking it. Next steps in the research will test the actual filtration capability of the peeled membranes in actual wastewater.

Second use for old reverse osmosis membranes: wastewater treatment

Abstract This paper describes some tests made with old seawater reverse osmosis (RO) membranes, modified to be reused as filtration membranes. These modified membranes were then used to reduce suspended solids in the tertiary treatment of municipal wastewater. The purpose of the tests was to establish the feasibility of recovering old (discarded) membranes in order to reduce industrial solid waste, as well as reducing costs in new filtration membranes. The active layer in the old RO elements was skinned off by recirculation of a potassium permanganate solution, which drastically reduced the salt rejection properties of the membranes, down to 2%. The elements so conditioned were then used as membrane filters inside pressure vessels, which had also been used in RO plants and recovered for a new purpose. The pilot plant was designed to reduce both suspended and dissolved solids from wastewater secondary effluent, with a production capacity designed for 6 m3/h. The reused membranes proved capable of reducing suspended solids (93.9% average reduction in turbidity), and the overall separation efficiency reached 97%, The RO unit reduced 98.5% conductivity at a 60% recovery ratio when operating at 32 bar. The filtration membranes developed a high pressure drop, and must be cleaned quite often, both by flushing at high recirculation ratios and by chemical cleaning. The cleaning procedures must be optimised and reduced to a minimum.

Electrodialysis desalination designed for wind energy (on-grid tests)

An electrodialysis desalination plant has been set up and tested while driven corn an off-grid wind-energy system. Prior to analysing the behaviour of the system off-grid, a thorough knowledge of the operational modes and constraints is required. That information is obtained through a number of tests done on-grid. This paper presents some results obtained while operating on-grid in a work carried out within the framework of a wider scope project, located in Gran Canaria Island (Spain). The main goal of this project is to test and identify the most suitable desalination systems for connection to the above-mentioned medium-scale off-grid wind farm. Afier preliminary modelling using proprietary software, a number of on-grid tests at different flow rates and futed feed conductivity were carried out. The results include curves for the relationship between voltage applied and product conductivity (between 100 and 280 pS/cm), as well as unit energy consumption (from 1.48 to 2.32 kWh/m3). The data gathered showed that EDR can easily adapt to a variable power supply such as wind energy, and they were used later in off-grid tests. Keywortis: Electrodialysis; Brackish water; Wind energy; Modelling; Control

Cleaning tests for seawater reverse osmosis membranes

Membrane fouling is a frequent problem in most seawater reverse osmosis units, particularly when raw water is drawn from an open sea intake. Membranes tend to foul slowly, which makes it necessary to perform periodic cleanings with physical and chemical products, which in turn produce a decrease in the useful life of the membranes. The specific objective of the research was to identify a rational chemical cleaning method and also to select suitable cleaning agents and their dosages. A number of experimental cleaning tests were carried out on a pilot plant with membranes previously used, and fouled, in a large seawater desalination plant (Las Palmas III). The procedure included performance tests before and after cleaning, and the cleaning itself, with acid and basic chemicals, as well as detergents. The results are presented in terms of four parameters to correlate the cleaning effect on the membranes: product water, conversion factor, salt rejection and turbidity. The cleaning agents that proved most effective for increasing product water flow rate were solutions such as basic chemical with detergent, acid with detergent, and also cleaning in two stages: hydrochloric acid plus caustic soda. Meanwhile the cleaning procedures that improved the salt rejection were in two stages: acid with detergent and basic with detergent.

Reuse and minimization of desalination brines: a review of alternatives

Abstract Desalination is currently one of the available solutions to the problem of the lack of water present in most countries due to the global increase in water demand. One of the desalination plants main problem is the large amount of reject brine produced, which can very often severely damage the receiving environment. Nowadays, the management of this residue is based mainly in the dilution prior to the discharge. However, there are many emerging alternative technologies that can be combined to achieve a minimization and valorization of brine and thus an appropriate management. This article introduces and describes briefly the set of available techniques. Each alternative has been assessed in terms of technology, economic and environmental merits, resulting in a numeric table.

Experience in desalination training and know-how in the Canary Islands☆

Abstract A brief review of desalination in the Canary Islands has been presented, from the point of view of technological status: training capabilities, as well as expertise developed. Thus, under-graduate and postgraduate (MSc, PhD) courses at the University are described, both on desalination and on environmental engineering. Also, a number of non-academic training programs are described such as desalination, water treatment, biological control.