DHV Water BV, P.O. Box 484, 3800 AL Amersfoort, The Netherlands, Tel: +31 33 4682497,
Fax: +31 33 4682301, Email: A.Giesen@WA.DHV.NL
An advanced alternative for phosphate removal by conventional precipitation is to apply crystallisation. DHV, a multi-national group of consulting engineers and general contractors with headquarters in The Netherlands, has developed and patented a fluid-bed type of crystallizer in which phosphate is removed and recovered from the wastewater while phosphate pellets with a typical diameter of 1 mm are produced. The major advantages of the crystallisation technology, the so-called Crystalactor®1, are that (1) the installation is compact, (2) phosphate pellets with a high-purity are produced, (3) the produced pellets have an extremely low water content (5% to 10% moisture) and (4) the pellets can be reused. Up to now Crystalactor plants were constructed for the removal/recovery of heavy metals, phosphate, fluoride and the softening of drinking and process water.
1The Crystalactor® is the registered trademark of the pellet reactor systems developed by DHV Water BV, Amersfoort, The Netherlands, for water treatment.
1 CONVENTIONAL PHOSPHATE REMOVAL
In municipal water applications and many industrial unit operations phosphate-polluted wastewater is generated.
In general, conventional phosphate removal techniques are applied for the wastewater treatment. These conventional processes are based on the phosphate precipitation as calcium or iron salt or fixation in activated sludge. These processes unfortunately generate huge amounts of a water-rich sludge which has to be disposed off at continuous increasing costs. To minimise disposal costs, the sludge is often mechanical dewatered prior to disposal. Typically, even after dewatering the water content of the sludge still amounts to 60% to 85% and a relatively large part of the disposal costs comes from the expensive disposal of water. Due to high water content and the low quality of the waste sludge, reuse of phosphate is not an economic attractive option. Furthermore the operation of mechanical dewatering equipment is often troublesome. Also the area requirement for conventional phosphate precipitation processes is relatively high because of the four process steps are performed serially. They are (also refer to Figure A):
2 CRYSTALLISATION IN A CRYSTALACTOR®
An advanced alternative is to apply crystallisation instead of precipitation. The Crystalactor®, a fluid-bed type of crystallizer, has been developed for this purpose. Instead of bulky sludge, this process generates high purity phosphate crystal pellets that can be re-used in many ways. Recovery of phosphate because more and more important since it is a sustainable solution to the environmental problems related to the mining and processing of natural phosphate resources.
The Crystalactor enables phosphate removal and recovery by means of several process routes. The most important routes are:
2.1 Process description
Actually, the chemistry of
the process is comparable to the conventional precipitation. By
dosing a calcium or magnesium salt to the water (e.g. lime, calcium chloride,
magnesium hydroxide, magnesium chloride), the solubility of CP, MP, MAP
or KMP is exceeded and subsequently phosphate is transformed from the aqueous
solution into solid crystal material. The primary difference with conventional
precipitation is, that in the crystallisation process the transformation
is controlled accurately and that pellets with a typical size of approx.
1 mm are produced instead of fine dispersed, microscopic sludge particles.
The Crystalactor® is a cylindrical reactor, partially filled with a suitable seed material like sand or minerals. The phosphate-containing wastewater is pumped in an upward direction, maintaining the pellet bed in a fluidised state. In order to crystallise the phosphate on the pellet bed, a driving force is created by a reagent dosage and sometimes also pH-adjustment. By selecting the appropriate process conditions, co-crystallisation of impurities is minimised and high-purity phosphate crystals are obtained.
The pellets grow and move towards the reactor bottom. At regular intervals, a quantity of the largest fluidised pellets is discharged at full operation from the reactor and fresh seed material is added. After atmospheric drying, readily handled and virtually water-free pellets are obtained.
2.2 No Residual Waste
A major advantage of the
process is its ability to produce highly pure, nearly dry phosphate pellets.
Table A shows the typically characteristics of the pellets in comparison
with precipitation sludge.
|Parameter||crystallisation in pellet reactor||conventional precipitation|
seed material content
CP, MP, MAP, KMP-content
|round pellets 0.8 - 1.0 mm 1 - 5 % < 5 % 90-98 %||sludge 60 - 85 % (after dewatering) - 20 - 30 % (after dewatering)|
No copious amounts of waste sludge, but compact reusable
Due to their excellent composition, the pellets are normally recycled or reused in other plants, resulting in no residual waste for disposal. Several reuse options are:
Phosphate processing industries have expressed interest in reusing Crystalactor pellets since it shows to be an attractive and clean (low in heavy metals) secondary phosphate source.
In the rare event that pellets have to be disposed of by other means, the advantage of low-volume secondary waste production still remains: water-free pellets, not bulky sludge.
2.3 Capacity and effluent concentrations
The reactivity of phosphate is reflected in the crystallisation process and high reactor loadings can be applied. The reactivity for MP, MAP and KMP is even substantial higher (factor 3-5) than for CP.
Depending on the pH and the calcium or magnesium dosage rate, phosphate can be removed from the wastewater down to low concentration levels. The phosphate concentration in the effluent from a pellet reactor when treating a typical wastewater stream depends on the applied process route. With the CP-route effluent concentration below 0.5 mg P/l can easily be obtained, while the other routes result in effluent containing typically 5-10 mg P/l. If the Crystalactor is applied in a side-stream, as is the case in combination with biological phosphate removal, the actual effluent quality discharged by the Crystalactor-unit is less important. In this case the crystallisation capacity determines the overall performance.
As result of the high reactor capacity, the high surface loadings (40-120 m/h) and since the coagulation, flocculation, separation and dewatering processes are combined into one by the crystallisation process, the unit often is compact (refer to Figure A).
Figure A: The four steps found in conventional treatment processes are combined into one by the crystallisation process
In general the crystallisation process enjoys a substantial higher selectivity than conventional precipitation. This is caused primary by the selectivity related to the crystal structure and secondly by the fact that adsorption of impurities to the phosphate sludge flocs is minimal.
2.5 Process parameters
The efficiency of fluoride removal for the pellet reactor depends upon the following three process parameters
The pellet reactor effluent contains dissolved phosphate and suspended micro-crystals from nucleation. Nucleation is effectively minimised by the particular construction of the crystallizer and the choice of the appropriate degree of supersaturation. The dissolved phosphate concentration is fixed by the solubility product, the ionic reagent concentration and the process pH. This means, that the desired phosphate effluent concentration can be obtained by selection of the pH and reagent dosage. In practice at the optimal process pH, an overdose of 0.5-5 mol/m3 is applied
At a given pH and overdose, the degree of supersaturation depends only upon the phosphate concentration of the wastewater. The phosphate concentration at the bottom of the reactor has to be maintained below a critical value in order to prevent primary nucleation. Moreover, the mechanical strength of the crystals decreases with increasing supersaturation. In practice, it has been observed that negligible nucleation occurs at a phosphate concentration of 25-125 mg/l P. This concentration is obtained in the pellet reactor by the correct selection of the circulation ratio, irrespective of the phosphate concentration in the wastewater.
Hydraulic reactor load
The hydraulic reactor load is the supernatant liquid velocity in the pellet reactor. This hydraulic load has to be selected in such a way that the pellet bed is fluidized. An increase in hydraulic load will result in an increase in secondary nucleation. In practice, good results are obtained for phosphate crystallisation with a hydraulic load of 40-75 m/h.
3 EXAMPLES OF APPLICATIONS
3.1 Phosphate recovery food industry
In the food industry waste waters with a high organic load are released. The waste water of a potato processing plant of AVEBE is treated in an anaerobic biological reactor because of the low sludge production, the low energy consumption and the biogas production. The effluent is polished in an aerobic biological treatment plant. Cost effective phosphate removal by struvite crystallization in the Crystalactor® was tested on semi-technical scale. MgCl2 and NaOH solutions were dosed into a part of the effluent of the anaerobic stage and in a fast reaction strong NH4MgPO4 crystals were formed. No filter is required because of the high crystallization efficiency and the fact that rest phosphate uptake takes place in the aerobic stage.
Phosphate recovery AVEBE
A flow of maximum 150 m3/h with 120 ppm PO4-P was succesfully treated in a Crystalactor® with a reactor diameter of 1.8 m).
The effluent contains about 10 ppm PO4-P and the pH is 8-8.5. The NH4MgPO4 pellets are reusable as "slow release fertilizers". The advantages of the Crystalactor® for AVEBE were:
3.2 Phosphate recovery municipal wastewater treatment plants
In 1988 the first full-scale application has been realised at the municipal wastewater treatment plant of Westerbork, The Netherlands.
Phosphate removal plant Westerbork
The plant operates successfully and removes phosphate below 1 mg/l P from the effluent of the biological section. No sludge is produced and the pellets are re-used by the phosphate processing industry. Since phosphate-free detergents are used in Dutch households, the phosphate concentration in raw municipal wastewater has decreased importantly. Direct phosphate removal from the effluent by the Crystalactor® is not economical attractive anymore and the plant was closed.
For these lower inlet concentrations a combination of biological phosphate removal and the Crystalactor® as is applied at the municipal wastewater treatment plants of Geestmerambacht (230.000 p.e.) and Heemstede (35.000 p.e.) is more attractive. The process set-up is as follows: - a part of the return sludge is pumped to an anaerobic tank where acetic acid is dosed (other lower fatty acids are also possible);
- phosphate is released by the sludge in this anaerobic tank;
- the sludge is separated from the supernatant by a gravity-thickener in Geestmerambacht and a decanter in Heemstede;
- the thickened sludge is returned to the aeration tank where it takes up phosphate again;
- the phosphate is recovered from the supernatant by Ca3(PO4)2 crystallization in the Crystalactor®; since the Crystalactor® is located in a side stream no filter step is required;
- the effluent of the Crystalactor®
is returned to the aeration tank.
4 OTHER APPLICATIONS
The pellet reactor crystallisation technology is not only applied for phosphate recovery, but also for water softening, fluoride removal and heavy metal recovery. In principle all crystalline salts can potentially be removed from wastewater. As shown in table C, there is an extensive experience in removing most heavy metals and major anions, and the number of applications continues to grow. Metals are generally removed as hydroxide, carbonate or sulphide compounds.
Table C: Periodical system showing pellet reactor experience
In some cases it has proved to be attractive to form metal phosphates. Anions are usually removed as calcium salts. Occasionally it is more desirable to form complex salts. For example, phosphate can be removed as NH4MgPO4 while simultaneously reducing the wastewater nitrogen content. The following picture shows some samples of produced pellets.
Pellet reactors for softening of drinking water, Municipal Drinking Water Company of Amsterdam, The Netherlands
Capacity: 8,500 m3/h
Pellet reactor for nickel and aluminium recovery,
5 ENVIRONMENTAL SOUND OPERATION
Municipal wastewater treatment plants and commercial industries need cost-effective, compact and reliable technology to reduce waste emissions. Moreover, this technology has to provide a sustainable solution to the problem of avoiding secondary emissions. Secondary emissions such as waste sludges represent a growing environmental liability for those producing them. More important, they will increasingly be subject to ever-rising charges levied by the authorities, and ultimately their disposal will be prohibited altogether.
Consequently, industry has embarked on a new strategy to tackle environmental load problems. Alongside waste recovery - often referred to as reuse or recycling - waste prevention is now a key feature.
The Crystalactor® offers a sustainable solution to above mentioned problems and combines an environmental sound production or wastewater treatment with attractive economics.