|M.Sc Student||Radu Irina|
|Subject||Study of Pretreatment of Secondary Wastewater in|
|Department||Department of Chemical Engineering||Supervisors||PROFESSOR EMERITUS Raphael Semiat|
|PROFESSOR EMERITUS David Hasson|
|Full Thesis text - in Hebrew|
Usage of secondary
treated sewage for reducing the water shortage problem has increased in Israel
and all over the world. There is a growing demand for high quality secondary treated wastewater, purified from
organic and microbial contaminations by a UF/MF process and from dissolved
salts, by an RO process. One of the problems in the RO process is the
presence of phosphate ions in wastewaters. Calcium phosphate is a sparingly soluble salt which can readily
precipitate on the membrane during the concentration process
accompanying permeate withdrawal. Currently available anti- scalants are not sufficiently
reliable to provide effective inhibition of phosphate scales in the RO process.
objective of this research work was to develop a novel simple method for
removing phosphate from wastewater fed to the RO process. Calcium phosphate is readily precipitated in an alkaline environment.
The principle of the proposed technique is to create an alkaline environment by percolating the feed water
through an MgO granular packed bed.
Beds of such granules are commercially available and widely used for
neutralizing acidic waters. The primary advantage of the proposed technique is
in providing a relatively simple process for coping with the calcium phosphate
goals of the research program were to obtain data on the efficiency of
by a magnesia packed bed, to characterize the need for periodic cleaning of the
bed and to develop a backwash technique
enabling effective regeneration of a clogged bed. The experimental system was
designed to enable scale precipitation in continuous flow through two columns connected in series. The feed solution was tap water (Talk = 117.5-170 ppm as CaCO3, Ca = 50-64 ppm) with and without phosphate addition
= 4-11 ppm).
The dimensions of each column were: diameter of 98 mm and height of 1800 mm, bed height of 500 mm.
The first column contained 4-4.5 kg of granular MgO particles. The strong alkaline environment created by the dissolution of magnesia served to precipitate two scaling components from the feed - calcium phosphate and calcium carbonate. Three kinds of magnesia having different chemical composition and grain sizes were tested:
The second column contained 4-4.5 kg of limestone particles, 3 to 6 mm average size, of a chemical purity of over 99.5% CaCO3. The purpose of this column was to complete the precipitation process and to filter out precipitated particles. The experiments were carried out at feed flow rates providing residence times of 8.8 - 44 kg ⋅ min/ L in the magnesia column. The main efforts were centered on characterization of the effectiveness of the various types of magnesia particles, on study of the loss of bed activity through decay of the pH, on tests of various bed regeneration techniques and on clarification of possible mechanisms causing vigorous deactivation of the magnesia bed.
The main findings of the research are as follows: