(215q) Investigating the Performance of a Continuous Electrocoagulation Unit for the Combined Removal of Nitrate and Phosphate Ions From Wastewater

The presence of excess nutrient materials such as nitrates and/or phosphates in wastewater causes the well known eutrophication phenomenon, which is the depletion of oxygen in such water as a result of algae growth due to the presence of such nutrients in high concentrations. Nitrates are relatively stable and highly soluble ions with low potential for co-precipitation or adsorption. Such properties make treatment of these ions for their removal from wastewater difficult, complicated, and expensive. Existing methods of removing nitrate from wastewater include ion exchange, biological decomposition, chemical treatment, reverse osmosis, electrodialysis, and catalytic denitrification. Although Ion exchange is a very efficient process, it is however fairly high in capital and operating costs with undesirable high residual constituents such as chlorides and bicarbonates in the treated water, which must be removed prior to consumption. Biological decomposition is a stable and extremely effective process in reducing nitrate by nearly 100% without using any chemicals. Unfortunately, this process is generally time consuming, limited in the temperature range, very costly and requires extensive maintenance. Therefore, in most cases it is utilized only in treating waste water of sufficiently high original nitrate concentrations.

Current employed phosphorus removal techniques include chemical treatments such as adsorption, chemical precipitation, ion exchange and electrodialysis, hybrid systems based on fly-ash adsorption and membrane filtration, and electrocoagulation. Among these methods, adsorption, and chemical precipitation are the most widely used for phosphate removal. Phosphate removal from aqueous streams is based on the conversion of soluble phosphate to an insoluble solid phase, which can be separated from water by means of sedimentation or filtration. In wastewater applications, the most common and successful methods to precipitate phosphate involve the use of dissolved cations, such as Al³⁺, Ca²⁺, Fe³⁺ and to a lesser extent Fe²⁺. It was found that when iron and aluminum are present in water, FePO₄ and AlPO₄form at a low pH range below 6.5, while at a higher pH range (above 6.5) iron and aluminum increasingly convert to oxides and hydroxides. However, precipitation of phosphate with calcium as apatites and hydroxyapatites at higher pH is more ideal for phosphate removal.

During the past two decades Wastewater treatment using electrochemical technologies has gained prominence. It has found wide applications in water treatment and metal recovery from wastewaters produced in various industries, such as tannery, electroplating, diary, textile processing, oil and oil refineries. In certain wastewater treatment applications, such as those involving refractory pollutants, electrochemical technologies are in many instances considered the best treatment choice.

Various types of reactors have found applications in electrochemical wastewater treatment processes. These include basic reactors, such as tank cells, plate and frame cells, and rotating cells, as well as, complicated three-dimensional reactor systems like fluidized bed, packed bed cell, or porous carbon packing cells. Rotating cathode cells are used to enhance mass transfer from the bulk fluid to the electrode surface and to remove the deposited metal powders from the cathode. A pump cell is another variant of a rotating cathode cell, which uses a static anode and a rotating disk cathode with a narrow spacing between the electrodes that allow for the flow of the effluent wastewater stream. Dissolved metals are electrically collected and scraped as powders. Lately, a continuous flow reactor based on a bipolar electrocoagulation-oxidation combined with electroflotation (ECEO–EF) was developed and tested for the removal of phosphate and ammonia under different operational conditions of pH, voltage, and detention time.

In this work we investigated the performance of an electrocoagulation reactor for the removal of phosphate and nitrate ions from wastewater using monopolar vertical iron electrodes. Experimental results show that phosphate and nitrate removal efficiencies are improved by increasing the solution flow rate and the current density while decreased by increasing (N/P) ions ratio and by increasing the initial concentrations of these ions. Experimental results further show that the electrocoagulation process can be described by a first order rate equation for the removal of both phosphate and nitrate ions. A correlation for the effect of solution flow rate on the rate of mass transfer for both nitrate and phosphate ions were also developed, which can be highly useful for the design of such electrocoagulation reactor for phosphate and nitrate mixed ions removal from industrial wastewater.