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Membranes in the Treatment of Leachate

In this article Steve Last of Enviros provides a personal view.

Membrane plants are increasingly popular for water treatment, and have been used in municipal water supply in the UK for at least the past 12 years.

As such it is natural that there will be considerable interest in the use of membrane technology for leachate treatment, and this is supported by the regular appearance of membrane related search terms which we see in the search log on this web site.

However, we are not aware of any membrane plants which are currently operational at municipal waste landfills in the UK, or Ireland.

The term “membrane” covers a wide range of membrane types and pore sizes. However, most fall into two main categories:-

• Nanofiltration (NF) and Reverse Osmosis (RO): These membranes take the form of either conventional spiral wound modules, or tubular modules which are operated in a cross-flow configuration, and cleaned mechanically (eg the FYNE process). Very approximately 25% of the feed flow is returned as a concentrated stream and must either be further concentrated to render it suitable for disposal off-site, or returned to the body of the wastes.
• Full Flow Ultra Filtration (UF) or Microfiltration (MF): These plants are based upon hollow fibre cassettes or modules, and are full flow (ie do not produce a discrete concentrate). These plants are also referred to as Membrane Bioreactors (MBRs), because they operate on the basis of an initial biological nitrification stage, and the effluent from that stage is drawn off subsequently via the membrane.

In both processes chemicals are used for regular automated cleaning to minimise fouling (clogging) of the membranes. The chemicals used for cleaning vary from one supplier to another, but normally include a combination of acid, caustic soda and hypochlorite solutions.

The main advantages often hoped for in the use of membranes for leachate treatment, and which if realised would herald their widespread adoption, are:-

1. smaller reactor sizes when compared with conventional SBR leachate treatment plants;
2. rapid portable installation, and the almost “instant” commissioning afforded by the adoption of a physical process, as opposed to a biological process which requires time for the build-up of an acclimatised sludge biomass;
3. energy savings when compared with conventional SBR systems;
4. odour and foam free in use;
5. highly automated system requiring very low operational input.

While (1) appears to be achievable but may invoke foaming problems, (2) is only theoretically possible in the case of NF or RO, if no biological pre-treatment is adopted. It is also the view of the writer that for both types of mernbrane process items (3), (4), and (5) above, remain to be consistently demonstrated in the treatment of UK leachates. Full flow systems usually simply comprise a biological nitrifying reactor SBR which, instead of undergoing a settlement cycle, utilise a UF or ME membrane system to provide a continuous throughput, rather than the cycle adopted in an SBR of the sort Enviros so often provide. Even in the case of Nanofiltration and Reverse Osmosis, prior biological pre-treatment is normally carried out to suppress fouling.

Why does the writer hold this somewhat negative view of the current membrane technologies?

The answer lies in three main concerns:

1. Fouling (the irreversible clogging of membranes) which is minimised by the primary pre-treatment of leachate; cannot be prevented entirely, and remains a problem which is worse for leachates than for potable water treatment, due to leachates being normally significantly more contaminated than for example, municipal water supplies, and indeed leachate is normally substantially stronger than sewage. It has become evident that membrane performance is limited by the tendency of these membranes to foul. Fouling causes direct cost implications in increased staff maintenance time, increased use of cleaning, and higher than anticipated backwash/chemical volumes. Back pressure increases require more energy use, to over come the actual pressures required to maintain the flow in the face of fouling.
2. Loss of membrane integrity (ie leakage) through the membrane, and the high cost of membrane replacement. Membrane elements which leak can rapidly cause the overall effluent quality to deteriorate as the “concentrate” stream enters the permeate product, and for strong leachates this effect is amplified, with only a very small loss of integrity capable of making a big impact on quality. The hollow fibres used in MBRs do break on occasions and can only be repaired when found by time consuming manual inspection. The membrane modules used in NF and RO similarly fail over time and identifying individual failed units requires specialist skills.
3. Difficulties regarding sustainability, and likely compliance with the IPPC requirements for the adoption of Best Available Techniques (BAT), especially for NF, and RO techniques. It is not possible to comment authoritatively on BAT for leachate treatment, as suitable guidance has not yet been published by the UK Environment Agency. The writer has not seen the draft guidance currently being prepared for the Agency by Enviros. It is nevertheless self evidently not BAT if no significant treatment actually takers place within the process. If the concentrated pollutants are simply returned to the landfilled wastes, sustainability is seriously in question. In any event “recirculation” in all its forms is problematic in the context of the EU Landfill Directive, and the banning of all liquid waste disposal to landfill in EU landfills, as from July of this year.

Much research is being undertaken currently to reduce and, if possible, prevent fouling, and particularly the irreversible fouling which occurs even after a rigourous cleaning regime is put in place. Membrane technology is improving rapidly, and will continue to do so.

(c) Enviros 2007