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:
Hydrolit Mg I and Mg II of Akdolit Co. Germany, 70-75%
MgO, particle size of 0.5-2.5
mm for Mg I and 2.0-5.0 mm for Mg II.
Dead Sea Periclase Co. Israel, 99% MgO, particle size of 2-2.6 mm
Corosex of Clack Corporation USA, 97% MgO, mean particle size of 1.4 mm.
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.
main findings of the research are as follows:
characterization of the MgO column:
Beaker tests indicated that the magnesia
phase governing the solubility of all tested
particles is Active Magnesia and not Brucite or Periclase.
The outlet magnesia solubility in
continuous flow experiments, expressed bythe saturation ratio index of Active
Magnesia SIact, was of the order of 0.11 when feeding distilled water to the bed
and of the order of 0.01-0.06 when feeding tap water.
phosphate and carbonate scale removals:
particles were found to be most effective in precipitating calcium phosphate and calcium
A very high alkaline
environment, of pH levels in the range of 10-11, is created by the magnesia bed.
Under these conditions, almost all the phosphate is precipitated
(residual PO4 concentration below 0.2 ppm), and 40-50% of the CaCO3 is co precipitated with the
The limestone column
was found to have a negligible contribution to the precipitation
Deactivation of the
- The potential of calcium phosphate and of
calcium carbonate precipitation markedly affected by the pH level of the
solution. The effectiveness of the magnesia
bed in precipitating the scaling salts was found to deteriorate gradually due to a continuous decay in the pH
level generated by the magnesia particles.
- An unexpected result
of this research was the vigorous deactivation of the magnesia bed that resisted all
regeneration techniques used in media filtration beds.
- The experimental data indicated that the
main parameter governing the decay in bed
activity is the total amount of treated solution. The addition of calcium phosphate and
calcium carbonate seed crystals to the magnesia
bed did not have an effect on the rate of bed deactivation.
Bed deactivation mechanisms.
Microscope (SEM) photographs of used magnesia particles showed significant
coverage of the particles by adherent crystallites.
Spectroscopy (EDS) analyses revealed that significant amounts of calcium phosphate and calcium carbonate
were present on used magnesia particles.
The failure of
vigorous mechanical backwash efforts, by water and air, to dislodge the
precipitated scale compounds from the bed, indicates that the crystallites observed on the
magnesia particles were attached by strong chemical bonds.
A literature search did not reveal
information on the phenomenon of strong chemical
bonding between scaling species and MgO particles. This unexpected phenomenon merits elucidation.
The main advantage of phosphate removal by
percolating a solution through a magnesia bed is the operational convenience of
avoiding the need of a chemical dosage
system. The present work has shown the drawback of a strong deactivation of the
magnesia particles by adhering scaling compounds. This drawback might be overcome by using a fluidized bed of magnesia
Kaneko and Nakajima
(1988) studied phosphate precipitation in a pilot plant containing a fixed bed of
magnesia particles. They were able to obtain a very high phosphate removal for a very long
period by controlling the feed pH through dosage of a NaOH solution. The need to incorporate an
alkali dosage system to the magnesia bed, detracts from the attraction of the
magnesia bed technique.