A zero energy treatment solution that not only filters, but removes nutrients, metals, and organics – The technology we are talking about in this editorial is by no means new to science; in fact it has been around since the dawn of time, well actually just after then. What could it be you may ask, the answer is plants. Not the mechanical type with pumps, valves and aerators, but rather the green growing type.
Now before you stop reading because you saw the words “green and plant”, and the mere mention of such words might suggest this is going to be another green washed editorial, I ask that you briefly put aside your prejudices as there is one main benefit of this technology that will no doubt be of interest – reduced costs.
The treatment solution in principle is quite straight forward. Construct shallow dams (which we call cells), fill them with various species of plants which thrive in water (and don’t mind a bit of salt), and then run dirty wastewater in one end to achieve clean water out the other end. Sounds simple? You’re right, it is – kind of. What is not so apparent is that there is actually a significant amount of complexity going on beneath the surface. The plants, their roots and all the dead leaf litter provide a complex structure that entraps particulates (be it soil or coal particles, algae, or clays) and other contaminants. They also create a great home for different microbes to grow, which consume the nutrients and organics in the wastewater as a food source. The result is clean water from a technology that is only powered by the sun (and the contaminants in the wastewater). We describe this planted system as an engineered wetland.
The main applications in the mining industry where engineered wetlands could be effectively used are:
- Delivery of sewage treatment that has lower cost and less risk of breaking down in remote locations
- Coal Seam Gas water treatment: a process to remove contaminants prior to treating with reverse osmosis or to naturalise the treated water prior to discharging it to the environment.
- Mine water treatment: a cost effective process to filter and stabilise water prior to environmental discharge.
- The challenges and solutions specific to these applications are now discussed.
Sewage treatment – A more reliable, lower cost solution
In a previous role I had working for a Membrane Bioreactor company, I remember coming across an industry example about a client who had purchased a membrane based package sewage treatment plant for a mining camp in remote north western Australia, from another supplier. The problem the client had was that the membranes kept blocking (probably due to insufficient access to experience operators combined with difficult water quality). Eventually, the problem got so bad and caused so much frustration, that they ripped the membranes out and ran the package plant as a basic aerated reactor. This may not be the situation for many installations, but it is not an isolated example. So often, when package treatment plants are installed in harsh remote environments, what originally looks good on paper ends up causing more problems than what it aims to solve. The more complex the treatment plant, the more reliant you will be on specialist people to fix the issues, and for remote mine sites getting these people (and spare parts) can be extraordinarily difficult.
Sewage ponds combined with engineered wetlands are a treatment approach often overlooked because of its simplicity. It lacks a lot of the sales pizazz that comes with high tech mechanical package systems. The set up typically involves building a series of dams (called ponds or cells by process people) where wastewater travels from one cell to another and is progressively treated as it goes. An engineered wetland follows the series of ponds, to produce crystal clear water that is low in nutrients.
Using the latest design approaches, treatment ponds combined with engineered wetlands can achieve BOD<2 mg/L; SS<2 mg/L, and ammonia <0.2mg/L; better than what can be done by most package treatment plants. And believe it or not, it is not smelly.
Many regional councils around Australia already use ponds and lagoons for sewage treatment. The reason why these solutions are more appropriate for remote sites than mechanical package plants comes down to there being less things to go wrong. They still provide treatment if the power fails. They don’t get blocked. They easily can handle flow variations. They can be scaled up to meet growing needs (with the addition of more cells). They can operate in harsh environments. All of these aspects make a pond/wetland treatment solution for sewage treatment significantly more cost effective and a lower risk option than using standard mechanical package plants.
To demonstrate just how effective engineered wetlands can be as part of a sewage treatment process, we show you seven years of data provided to the EPA under licence for a 4ML/day regional sewage treatment plant that is coupled with an engineered wetland (see adjacent graph). Although the main mechanical treatment plant performs relatively well, the effl uent quality can be variable – due to power failures, mechanical failures, changes in flow etc. The engineered wetland on the other hand, provides very consistent good quality effl uent over the seven year period. Whether engineered wetlands are coupled with treatment ponds, or other more traditional sewage treatment processes, there are significant advantages to consider them as part of any treatment solution.
Coal Seam Gas water treatment – lowering mainstream treatment costs
For most people reading this article, I don’t have to tell them that coal seam gas (CSG) water is a major challenge. Nor am I going to tell you that engineered wetlands will provide a total solution to all your coal seam gas water problems. What I can tell you about is how engineered wetlands can combine with your existing CSG water treatment systems as a practical way to reduce overall operating costs.
At the heart of most CSG water treatment is reverse osmosis. Because of the sheer volume of CSG water that needs to be processed, any improvement in operating efficiencies, even if small, can have a significant impact on the bottom line.
The main purpose of using engineered wetlands is to cost effectively clean up the water prior to it being processed by the main RO/fi ltration technologies. The cleaner the incoming water, the less work the mainstream process has to do, and the lower the overall operating and maintenance cost.
One of the main differences between wetlands used for CSG water treatment and wetlands used for municipal applications is that the species of plants that are used for CSG water systems have to be salt tolerant. Several native marsh species have been identifi ed which would suitably serve the purpose.
A well designed wetland system will be able to treat raw CSG water so that suspended solids are less than 2mg/L and the bulk of the organic loading is removed, including BTEX and other toxic contaminants. To estimate wetland size, a general rule of thumb is 1 to 2ha per ML of daily flow, but for most remote locations this isn’t an issue. The large vegetated area will lose about 7-9% of the total flow to evapotranspiration, which may be a cost if 100% of the water is reused, but at the same time could be an advantage to reducing processing volumes.
And what about the capital cost to build an engineered wetland? This is going to depend on a lot of factors, but generally is about $100-$250k per ha. A site specific assessment and water quality modelling is needed to determine whether the cost would be at the upper or lower end of this amount.
Mine water treatment – passive solutions for remote locations.
The chemistry of mine drainage and run-off water can be quite complex – but on the whole is relatively well understood. The major issues of concern usually involve iron and other metals and acidity. Natural wetlands can offer a low cost solution to managing these waters, especially with regards to filtration, metal removal and pH stabilisation.
The Water and Carbon Group
We are a company that designs, builds, and maintains ecological infrastructure; engineered wetlands, treatment ponds, and native reforestation / carbon offset projects (which wasn’t talked about in this article). If the technologies discussed in this article are of interest, we can provide you with some early stage concept designs, performance estimates and budget costing. Please contact our National Business Development Manager, Ian Kikkert for more information – Ph: 07 3211 9997 Email: i.kikkert@waterandcarbon.com.au Website: www.waterandcarbon.com.au
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