Introduction/Overview on Sulphate removal system
The treatment system was developed so that it could be easily fitted to existing effluent treatment plants or a new plant built incorporating the effluent treatment within one plant. The technology has been developed so the effluent plant can remove all contaminants with a single settlement/solids-liquid separation and dewatering step.
The aim of the technology was to provide a method to allow the removal of sulphate below consent limits without producing a waste that required further treatment, the process was designed to be single stage and reaction time to be no longer than one hour. This is important as this enables the space required for the plant to be minimised.
The technology has an added benefit of producing an effluent which does not lead to scale formation e.g. limescale, that some effluents/plants have as a major problem. This is achieved as a direct result of the chemistry and does not require any additional chemicals to be added.
Applications already trialled
Surface Finishing of metals — coin production, anodising, electroplating in particular chrome plating.
Mining — waste water treatment
Chemical industry e.g. soap manufacture
Waste disposal industry.
Typical removal rates
Sulphate reduction by up to 99.7%
Chemical Oxygen Demand reduction by up to 55%
Heavy metals reduction by up to 99.7%
Phosphate reduction by up to 99.9%
Exact reductions are dependant on dosage rates.
- Single stage solids/liquid separation
- Ease of use for operators and maintenance
- Waste filter cake can be re-used.
Information on the sulphate removal technique.
The technique can be applied as an end of pipe technique or included within an effluent treatment plant. An example of this is within a treatment plant for e.g. chromium removal.
The technique involves the reacting of the sulphate with aluminium hydroxide chloride under acid conditions ideally <1.3. The quantity of aluminium hydroxide chloride that is required is proportional to the concentration of sulphate within the effluent. After mixing the effluent is neutralised with lime slurry or liquid lime. The optimum pH has been found to be 11.5. The effluent is then pumped through the ultrasonic reactor at a controlled rate.
The amplitude and power of the ultrasonic vibrations are controlled to optimise the removal efficiency.
Within the ultrasonic reactor the reaction chemistry and kinetics are altered so the calcium aluminium sulphate oxide is formed. This material is a very fine precipitate almost colloidal in nature and it has been found that flocculation is started by the addition of Epofloc L1-R (a heavy metal precipitant, which is a carbamine), not only does this ensure the removal of any heavy metals that are present it removes any excess aluminium. The material is flocculated using an anionic polyacrylamide. The resultant supernatant is clear and colourless.
The precipitant produced in the process has been analysed using X-ray fluorescence and diffraction and has been identified as a type of zeolite.
The diagram that follows is a P&ID of an existing installation, which includes chrome reduction with metabisulphite. The plant involves treating two separate effluent streams one being for chromic acid based effluent and the other phosphoric/sulphuric based effluent.
Achieved environmental benefits
The reduction of the following contaminants:
The resultant filter cake produced has a lower moisture content and is often non hazardous and can be disposed of in a suitable landfill site. It can sometimes be reused in the following applications:
As an alternative raw material in the cement industry
As a treatment material for paint wastes e.g. spray booth waste
In waste stabilisation/solidification
In treatment of contaminated soils.
A side effect of the process is the fact that the precipitate produced is fine and extremely slow to settle. This requires a settlement tank/clarify that is designed for this purpose often the settlement velocity is as slow as 0.18m/h. Care has to be taken with the addition of Epofloc Ll-R and flocculant as to ensure that the flocculated particles settle to quickly leaving behind fine particles of the calcium aluminium sulphate oxide.
The consumption of raw materials is as follows based on a flow rate of 5m3/hr, 10 hours a day, 5 days a week, with a input sulphate level of between 2,500 and 8,000 mg/1, normally around 5,000mg/l.
Aluminium hydroxychloride 1,300kg/wk
Liquid lime 18%w/v 4,000kg/wk
Epofloc Ll-R 5kg/wk
Anionic flocculant 3-5kg/wk
The electrical consumption of the ultrasonic reactor is 120-140 watts.
The filter cake production is 5-6,000 kg/wk.
The analysis in the table has been conducted by ICP for metals and ion chromatography for sulphate.
The plant is operated automatically, with the required pH values being achieved by use of digital dosing pumps and connected directly to pH controllers. The 4-20mA signal from the pH controller is input into the dosing pump so the dosing is proportional. The ultrasonic reactor is started and stopped on the flow through the system. The ultrasonic reactor has it’s own control unit for setting the amplitude and power. One consideration from an operational/maintenance perspective is the keeping clean of the pH electrodes. Since the efficiency of the removal process is affected by pH, regular cleaning of electrodes is required, in practice this can be achieved with auto clean electrodes.
The technique can be applied to new or existing plants. If the technique is retrofitted this often will involve adding some dosing pumps and the ultrasonic reactor. Assuming the settlement tank or clarifier is suitably sized. The technique can be applied to plants large or small, in the case of large plants it may require several ultrasonic reactors to handle the flow (normally only required for flow rates >40m3/hr).