Reverse Osmosis (RO)
Waste Water Treatment — (Membrane technology — Reverse Osmosis Plants)
Clarke Energy is the exclusive distributor of HAASE membrane plants for the treatment of contaminated waste water including:
- Landfill leachate
- Effluent from Mechanical Biological Treatment and Anaerobic Digestion plants
- Surface water
- Ground water
- Industrial effluent
With over 20 years of experience and innovation in the design, manufacturing, installation and commissioning of membrane plants, coupled with hundreds of thousands of hours of operation, Clarke Energy and HAASE have earned the reputation of delivering high quality, robust and reliable plants.
HAASE currently have more than 70 membrane systems in operation worldwide, which includes systems based on Ultra-Filtration (RF), Reverse Osmosis (RO) and a combination of both. The plants supplied are either bespoke design, optimised for their specific application or standardised. They are mobile, self contained and ready to operate plants.
What is Reverse Osmosis?
The RO process extracts clean water from a waste water of organic and inorganic contaminants.
The process exploits the natural phenomenon of osmosis, whereby if two aqueous solutions with different degrees of concentration are separated by a semi-permeable membrane, then water from the weakest solution will pass through the membrane to dilute the higher concentration solution on the other side. The process will continue until solutions on both side of the membrane display the same degree of concentration.
With RO, the process is reversed. Pressure is applied to the waste water against a semi-permeable membrane, forcing water molecules to pass through the membrane, resulting in the formation of a clean “permeate”. The majority of the solutes or contaminants are left behind forming the “concentrate”.
What is the Treatment Efficiency of a Reverse Osmosis Plant?
RO is the finest physical separation method known. In contrast to normal filtration where solids are eliminated from a liquid, RO succeeds in removing even solutes from a solvent.
Figure 1: Filtration range comparison.
Most physical, chemical and biological processes for waste water treatment are optimised for the removal or treatment of a particular contaminant or group of contaminants. The RO process, however, is indiscriminate as its retention efficiency is primarily dependent upon molecular weight and polarity of the contaminant molecules. The exceptional characteristics of RO membranes results in more than 99% of large molecules dissolved in the waste water to be retained. Even ions of valance 1 such as Na+ and Cl- are retained.
Most commercially available plants are constructed as two stage plants, with contaminant removal rates better than 99.6%. Where an unusually high strength waste water is treated or very stringent discharge consents apply, a three stage plant can be employed to achieve contaminant removal rates better than 99.98%.
Figure 2: Retention effect [%] against number of membrane stages.
Reverse Osmosis Plant Overview
Most commercial RO plants designed for the treatment of a waste water are of multi-permeate stage configuration, typically two and occasionally three stages. The first stage provides the majority of the treatment while subsequent stages “polish” the permeate further.
The plants use artificial, semi-permeable membranes of thin film composite construction. Such membranes have high salt rejection and display very high physical and chemical durability.
A variety of membrane module systems are available, including proprietary tubular modules, spiral wound modules, hollow fibre modules and disc tube modules. Standard spiral wound modules, hollow fibre modules and disc tube modules are sensitive to the presence of solids in the leachate. For this reason RO plants incorporate a pre-filtration stage using sand-filters and fine filters.
The membrane modules are mounted inside pressure tubes on racks, complete with interconnecting pipework and recirculation pumps. Leachate recirculation in each membrane block provides constant conditions on the membrane surface. The feed to a membrane must be of a sufficiently high velocity in order to avoid concentration polarisation and fouling effects that would decrease their efficiency.
RO plants are designed to provide as large a surface area of membrane as possible for a given treatment unit. Based on calculated flux rates of permeate through the membranes, values of between 13 and 15 litres of permeate per square metre of membrane per hour have been reported.
Membrane flux rates gradually reduce, therefore, they need to be washed intermittently with a solution of membrane detergent and permeate produced by the plant. There is no requirement for a fresh water supply. “Wash” cycles are generally managed automatically and their frequency is governed by the level of contaminants in the leachate and in particular those of calcium, BOD5, COD etc.
Figure 3: Two-stage RO container schematic diagram and interior.
Continuously working RO plants are fully automatic. Operating parameters are permanently recorded and displayed. Start and shutdown procedures occur automatically. In most cases remote plant operation is possible.
Reverse Osmosis Treatment of Landfill Leachate
RO is a well established technology for waste water treatment and water purification applications. Developments in membrane technology, particularly of spiral wound types, has also resulted in the wide spread use of RO on European landfills for leachate treatment.
The main application of RO treatment on landfills is:
- Permanent on-site treatment
- Supplementing existing alternative on-site treatment processes, for example:
- An SBR plant which is unable to treat sufficient volumes of leachate due to heavy rainfall or reduced treatment efficiency during the winter
- Providing temporary treatment whilst an SBR plant is out of operation
- An SBR plant that cannot satisfy the discharge consent conditions
- A more cost effective and environmentally sustainable alternative to tankering leachate off-site
Currently, there are over 100 plants in operation, either in isolation or in combination with biological treatment plants, some for more than 15 years. As a fine filtration process, RO is particularly well suited for leachate treatment as it is capable of removing greater than 99.9 % of BOD, COD, ammonia, metal, inorganic salt and List I and II substances.
During the late 1990’s a lot of RO leachate treatment systems were designed with an aerated lagoon in front of a 2-stage RO plant. The advantage of this configuration is that an aerated lagoon reduces the readily biodegradable ammonia, BOD5 and COD by its biological activity and the RO plant deals with the non-biodegradable components of the leachate.
Experience on European landfills treating “strong” leachate, i.e., ammoniacal-Nitrogen > 1000 mg/l, shows that a 75% permeate yield (ratio of permeate:influent) is typical. On weak leachates, yields of up to 90% can be achieved.
The permeate is suitably clean to be discharged without any further treatment. In most instances, the permeate would even satisfy surface water discharge quality. The concentrate is normally re-infiltrated into the landfill body or tankered off-site for disposal, for example at a sewage treatment works.
Figure 4: 100 m3/day RO leachate treatment plant in Laval, France
Figure 5: 200 m3/day rental RO leachate treatment plant at Calvert Landfill, UK, commissioned in June 2006
Figure 6: Two-stage RO plant with aerated leachate lagoon, direct permeate discharge and concentrate re-infiltration to landfill (120 m³/d, Rebat landfill, district of Amarante, Portugal, commissioned 2001) (Loeblich 2005)
Improving Water Quality Downstream of a Landfill
In most instances, the permeate produced by the RO plant will be cleaner than the water quality standard of the river or stream it is discharged into. Therefore, water quality downstream of a landfill can be improved as a resulting of operating an RO plant.
Figure 7: Visual comparison of RO permeate (left) and tap water (right)
Reverse Osmosis Plant Flexibility
- Very small footprint plant - fits in an ISO container
- Mobile asset. RO plant can be delivered to site on the back of a lorry and can be moved between landfills
- Small plants are exempt from planning permission in certain situations
- Modular plant – easy to upgrade and downgrade treatment capacity
- Available for short-term rental or purchase
As a physical treatment process, RO has a residence time of just 3 minutes, whereas for a conventional biological treatment process, like an SBR plant, it would be 10 to 15 days. Consequently, an RO plant has a very small footprint. This portability lends the process to much easier volumetric plant upgrades and downgrades, thus making it the only commercially available leachate treatment technology that is truly modular and mobile.
The small footprint of an RO plant means that very little infrastructure needs to be in place other than the hard-standing area for the plant and chemical storage tank. Installation and commissioning of such a plant will normally take 3-4 days.
A plant with a treatment capacity ranging between 25 to 200m3/day can be delivered in a 40 foot ISO container on the back of a lorry. If required, any plant can be upgraded to a 200 m3/day three-stage plant all within the same container. The plant can also be moved to another site, thus making it a mobile asset and suitable for hire or purchase.
As a mobile structure, self-contained, with temporary external connections and not being permanently fixed to the ground, RO plants are exempt from planning permission when used on a short-term basis.
RO Reduces Financial Risk for Landfill Operators
- RO plants can be rented before purchase to gain confidence in the leachate volume present in a landfill
- RO plant rental is cheaper than off-site leachate tankering
- The capital cost of large RO plants is considerably less than that of SBR plants
- The price differential when upgrading RO plants is very small, especially compared to SBR plants
- Plant depreciation is very short
- There is no plant decommissioning cost at the end of a plants useful life
RO plants can either be hired or purchased. The modular design of RO plants and the option to hire them removes the risk of capital investment and commitment to a wrong sized plant, an important consideration for landfill operators who often won’t commit to permanent on-site leachate treatment plants due to uncertainty about the volume of leachate in a landfill. For example, it is not uncommon for leachate treatment plants to run well below their treatment capacity within a few years of being commissioned. This is due to perched leachate tables and increasing difficulty of pumping leachate out of the landfill as a result of draw-down. Therefore, plant hire can be very attractive to a landfill operator in the first few years of leachate management in order to gain confidence in both the leachate volume present and its extractability before committing to plant purchase.
The capital cost of RO plants is also cheaper than SBR plants. Furthermore, when upgrading RO plants, the price differential is very small compared to SBR plants.
The absence of extensive civil works means that plant depreciation can be very short compared to the typical 15 to 25 years for a biological treatment plant.
There is no large decommissioning cost at the end of an RO plants useful life. Instead the plant can be moved to another landfill site.
Reverse Osmosis Vs SBR Treatment
The advantages of RO treatment compared to SBR treatment are listed below:
- Lower capital cost than SBR plant
- Produces the highest quality of effluent of any commercially available leachate treatment process
- Suitable for BOD, COD, ammonia, metal, chloride and List 1 and 2 substance removal
- Suitable for the treatment of new (acetogenic) leachate, whilst still producing a permeate that can satisfy surface water discharge quality
- If permeate is discharged to sewer, an RO plant will generate the lowest trade effluent charges based on the Mogden Formula
- Very small plant footprint if there are space constraints on site
- Very easy to upsize and downsize RO systems unlike SBR plants
- Very short commissioning period – typically 3-4 days
- Being a physical process, plants can be operated on a part-time basis, for example 5 days a week. When switched back on, permeate quality is resumed within a few minutes
- Permeate quality is unaffected by cold temperatures. In comparison, with SBR plants ammonia removal efficiency is commonly affected by winter conditions, which makes them seasonal performers
- As a physical process, RO is quite insensitive to changes in leachate strength, unlike SBR plants. Although changes in leachate composition will effect the quality of permeate, well designed plants will monitor this and adjust automatically either the throughput and/or yield ratio to compensate
Reverse Osmosis Vs Off-Site Leachate Tankering
Many landfill operators manage leachate levels in their landfill sites by tankering leachate off-site to sewage treatment works for treatment. This is the most common and simplest leachate management option, but also the most expensive option.
If a landfill site is tankering leachate, has leachate storage tanks and a discharge consent to sewer or surface water, then an RO plant can very easily be installed. This will be significantly cheaper than tankering. It is also a much more environmentally sustainable treatment option.
Concentrate Disposal and Management
One of the issues discussed in the use of RO systems for leachate treatment is the management of the remaining leachate volume, the concentrate.
Concentrate management and disposal options include:
- Re-infiltration back into the landfill
- Off-site treatment, for example at a sewage treatment works
- Further treatment on-site using a second RO plant to reduce the volume of concentrate which can then be disposed off-site for treatment. For example an additional high pressure 2-stage concentrate plant, to increase the permeate yield to nearly 90%.
- Secondary concentrate treatment processes, such as evaporation and dryers to reduce the concentrate volume further
The most cost effective option for a landfill operator has to be the re-infiltration of concentrate back into the landfill, which is widely practised on European landfills. However, in examples where an RO plant is being rented, even if concentrate is tankered off-site for disposal, this is usually still cheaper than tankering all the leachate off-site for treatment.
There have been concerns about the effects of re-infiltration of concentrate into the landfill body. It has been suggested that long-term most contaminants will inevitably reappear in the leachate. However, numerous scientific publications within the last two decades have shown that in many cases re-infiltration of concentrate is beneficial and should not be discouraged. Homogeneous moisturisation of a covered landfill body through concentrate re-infiltration can increase gas production and shorten the landfill after care period.
It has been reported that in general, leachate concentrate recirculation accelerates biochemical processes and provides a tool to deal with landfill-associated problems in a period when the landfill is managed, rather than in an unmanaged period of aftercare.
The German Bavarian Environmental Protection State Office ruled from its scientific review that there is no arguing against leachate concentrate infiltration. Landfill data showed that there was no increase in the strength of leachate components and an inhibition of biological reactions in the landfill body should not to be expected either.
In the UK the Environment Agency is preparing new guidance for assessing the suitability of a landfill for concentrate re-infiltration. This will be in addition to the general guidance already published in the Environment Agency’s Guidance for the Treatment of Landfill Leachate (2007), which sets the following criteria for concentrate re-infiltration into a landfill:
- Any predicted change in leachate concentration should be assessed
- It must be shown that the landfill is adequately engineered so that the concentrate does not cause pollution (particular attention should be given to the impact on groundwater)
- It must be shown that the leachate treatment system (RO plant) can adequately treat any predicted change in leachate quality resulting from the return of the concentrate
- Chemicals essential to the effective operation of the RO plant should be selected so as not to compromise the disposal of the concentrate
At WRG’s Calvert Landfill, which has been operating a 200 m3/day RO plant since June 2006, the EA has in principle decided to allow concentrate to be re-infiltrated, subject to the landfill operator satisfying the above criteria and assessing the risk of this activity.
References
Anonymous (1999). Press release, Haase Energietechnik GmbH, 24531 Neumuenster, Germany
Anonymous (2002). Bavarian Environmental Protection State Office, publication: Treatment of Landfill Leachate in Bavaria: Fundamentals, Research and Practice; Augsburg 2002
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