The most common contaminants removed by AWT
The issue of how to deal with the contaminated water created by modern industrial practices is one of the biggest issues facing a vast and diverse range of manufacturing industries. Here at AllWater Technologies Ltd we’re focused on using the most advanced technology to provide intelligent, cost-effective solutions to wastewater treatment in a responsible manner.
In Part 1 of our two part series, we cover the most common contaminants we remove for our clients and why it’s necessary.
Why Wastewater Treatment Matters
In today’s challenging environment, our clients who have to deal with these wastewater streams operate with dual pressures:
- Trying to achieve ever more stringent wastewater discharge consent limits
- Trying to reduce operational wastewater treatment
The issue has become more high-profile in recent times due to the growing environmental issues arising from the increase in global industrial effluent. In order to remain compliant, responsible and profitable, our clients come to us for help to design, install and maintain the most effective processes and solutions for their particular business.
Which Industries Require Wastewater Treatment Processes?
The requirements and obligations for the treatment of industrial effluent are wide-ranging, depending on the kind of manufacturing involved and local regulations regarding discharge. At AllWater Technologies Ltd, our clients come from a diverse range of industries including:
- Surface Finishing
- Chemical Processing
- Food & Beverage industries
- Metal Finishing
- Pharmaceutical & Biotechnology
- Other manufacturing processes.
For some of our clients their requirements can be fairly straightforward, while others require a much more complex system. We always carry out a detailed audit of each client’s business in order to create a bespoke wastewater treatment solution that’s individually tailored to their waste products and legislational obligations.
The Most Common Contaminants Industries Need to Remove
The types of contaminants that need to be removed from industrial run-off are as diverse as the industries themselves. A sustainable, cost-effective solution requires specialised processes and the wastewater treatment processes we employ for our clients include:
- chemical treatments (pH correction, Cr reduction, CN oxidation)
- conventional settlement and filtration systems
- membrane filtration (MF/UF/NF/RO)
- vacuum evaporation
- ion exchange for water recovery and selective removal of heavy metals
Some of the most common contaminants we remove for our clients in manufacturing industries are:
- Hexavalent Chromium
- Heavy Metals
Removal of Hexavalent Chromium
Although no longer as commonly used due to its carcinogenic properties, our clients in manufacturing industries that employ various metal finishing processes, including chromic acid anodising, still sometimes use hexavalent chromium. Although we’ve seen use of this biotoxic metal being phased out and new alternative finishing technologies employed, special dispensation is still available for its use particularly in the aerospace and defence industries (where strict standards are applied in some instances).
Our many years of experience with wastewater treatment have proven that the easiest way to remove hexavalent chromium is to reduce it into the trivalent form, after which it can easily be precipitated as a hydroxide.
- We automate the reduction of hexavalent chromium in a reaction tank where pH and reduction/oxidation (redox) potential are continuously monitored.
- In order to achieve rapid reaction conditions, we ensure that the pH is maintained below pH2.5, and sodium bisulphite is commonly used as the reducing agent.
- For us to determine the exact redox potential below which all the hexavalent chromium has been converted, it is necessary to carry out a calibration graph. Typically the end point for the reaction is between 250 – 350 millivolts (mV) as the reaction proceeds acid is consumed and requires replenishment.
- Once the reaction is complete, pH can be raised to a level where optimum precipitation of chrome hydroxide takes place at around pH 8.3.
- With careful pH control followed by flocculation and settlement, it is possible for us to achieve removal to low milligram per litre levels.
Removal of Heavy Metals
We can easily remove most heavy metals – such as nickel, zinc and copper – by precipitation as metal hydroxide. In order to do this, we need to achieve the correct pH range for adequate removal to low milligram per litre levels.
We can find this difficult to achieve where there is a mixture of metals with varying optimal pH precipitation points. Sometimes we can extend the pH range for various metals by using an alternative chemical reagent.
- For example, we sometimes use Calcium hydroxide (lime) instead of sodium hydroxide for extending the range at which chrome can be reduced to low levels. In addition, sometimes we can dose a coagulant such as a ferric based product to aid co-precipitation.
- In extreme circumstances we may use sulphide based chemistry (or a derivative) to precipitate a range of metals to very low levels, even in the presence of complexants which bind the heavy metals in solution, making them difficult to precipitate as a hydroxide. However, we typically avoid sulphide based chemistry in our wastewater treatment if at all possible, due to the toxic nature of most sulphide based chemicals themselves.
Removal of Cyanide
For our clients in industries that employ metal finishing chemistry, we find that cyanide is less commonly used now, however it is still regularly used when plating precious metals such as silver and gold.
- The most common method we used for removal of cyanide is to oxidise it using a strong oxidising agent such as sodium hypochlorite. In order to ensure that this reaction takes place rapidly, we carry out the process under high pH conditions: i.e. greater than pH 11.
- As with chrome reduction, we monitor the redox potential to determine when all cyanide has been destroyed. Typically this is at a redox potential greater than 600 millivolts. We can determine the exact end point on commissioning by measuring the redox potential and looking for breakthroughs of free chlorine.
Removal of Phosphates
The presence of phosphates in inland waters leads to eutrophication and, in turn, massive growth of algae and plants. This then leads to deoxygenation and then death of biological organisms such as fish.
This problem has been highlighted much more recently, with many of our clients in industries previously discharging phosphate rich waste streams now being asked to remove them at the source.
In municipal plants the breakdown of phosphates is achieved by bacteria, but growth in some population areas that hasn’t gone hand in hand with expansion of municipal treatment plants, has led to a notable increase in phosphates making it through to inland waters. This is why pressure has been put on our clients in the related industries to reduce the amount of phosphate discharged to drain.
Often it is not practical for an industrial user producing a phosphate rich waste stream to build and operate a biological treatment plant, so we need to find and employ an alternative wastewater treatment solution for those clients.
- We’ve found that phosphate can be easily precipitated to relatively low levels through use of a calcium rich reagent such as Calcium Chloride or Calcium Hydroxide (lime). Lime is more commonly used on larger installations, where it is generally cheaper but is often more difficult to use.
- In order to dose we either make it up from a powder as a slurry, or ( as it’s now more commonly available) as a high concentration liquid suspension. An example of this is Callic HS.
Removal of Sulphates
At a concentration above 1,800 milligrams per litre, sulphates can cause damage to concrete drains. More recently, drainage systems have been constructed from plastics and therefore this has become less of a problem for our clients in the related industries. However, for one reason or another, we’ve found that sulphate discharge consent limits generally seem to be decreasing across the board. This means for some of our clients it poses a problem, because sulphates can be particularly difficult to remove.
Although they will precipitate as calcium sulphate, it’s not possible to use the method to achieve a limit below 1,800mg/l. However, it is possible for us to precipitate sulphates to levels of less than 100 milligrams per litre if precipitated as an ettringite. In order to achieve this, we often employ proprietary chemicals with high pH.
- The precipitation of sulphate as an ettringite typically results in production of high levels of solids and a supernatant of high pH, which we then neutralise before it is discharged to drain. We do this by using a number of possible reagents, depending upon what is required to be achieved for the final effluent. In certain circumstances where a waste stream contains high levels of sulphates but little else, by choosing the right neutralising chemical at the end of the process it is possible to achieve a low total dissolved solids (TDS) stream.
Removal of chlorides from a waste stream by chemical means is extremely difficult and, while barium can be used for precipitation, we don’t generally consider it a suitable solution for our clients, due to the cost and toxicity.
Removal by physical means such as concentration on a Reverse Osmosis membrane results in a more concentrated, if reduced waste stream, which still requires disposal. For our clients that need to comply with low level chloride limits, we often find it necessary to consider the upstream process more holistically for them, to see whether an alternative product can be employed to reduce the level of chlorides in their waste.
Removal of Ammonia
The presence of ammonia in a waste stream resulting from a physico-chemical process is unusual and we may use alternative chemistry to remove its presence. For our clients where this is not possible:
- If we find high levels of ammonia are present we may be able to remove them by air stripping.
- For low levels of ammonia we may offer more exotic options, but depending upon where the waste stream is derived, treatment may result in other unwanted species such as halogenated hydrocarbons, so our experience has shown we must proceed with caution.
- More commonly, ammonia is removed through biological treatment systems.
Read on for Part 2 in the Series
In Part 2 of our expert series we’ll cover the cutting-edge technology we employ to provide the most responsible, sustainable and cost-effective wastewater treatment solutions for our clients: “Our Technologies for Removing Contaminants from Water”.
Get in Touch
If you have been tasked to source an effluent treatment system or to upgrade your existing plant, you can rest assured that not only will we design and install a technologically advanced system, but that you will benefit from our effective and responsive support team. With 24/7 telephone support and engineers on call across the country as well as equipment available for emergencies, we make sure your backups are in place and always on standby. Get in touch with one of our highly skilled engineers to discuss your needs.