Membrane technology is emerging either as a replacement or for uplifting the efficiency of various conventional technologies like filtration, softening, wastewater treatment, desalting of brackish waters, desalination of seawater and deionisation. Increasing polluting levels and salinity in freshwater systems demand the use of membrane technology in water treatment.
Many improvisations have taken place in membrane development making it more viable and feasible in terms of low energy consumption elements, high flux or permeate recovery through increased surface area, better configurations and materials of elements.
Many membrane systems have become more susceptible to fouling or other process failures with increased developments. Thus, one needs to be extra careful in deciding pre-treatment to RO membranes.
Table-1 illustrates types and causes of fouling and possible remedies to avert them.
One of the weakest features of RO membranes is that these are good to tackle invisible enemy (dissolved ions) but equally prone to visible ones, specifically particulate matter and surfactants. We have to look at how to opt for a faultless pre-treatment for various sources of RO feed water.
Right selection of pre-treatment may:
a) Increase life of membranes,
b) Decrease fouling potency and thus less cleaning of elements, and
c) Better performance both ways yield and operational costs.
Membrane technology is a fine engineering and it takes years to develop a particular membrane element; as a result, membrane cannot fail by itself. It is some other factor that makes it fail. Experience shows that many systems have failed or underperformed for the reason that a wrong assessment was made while considering pre-treatment for membranes.
Following are some critical considerations while designing an RO system:
- Source of water
- Environmental conditions of site
- Past trends of change in composition of source water and future predictions
- Detailed analysis report from a good laboratory
- Selection of appropriate pre-treatment ahead RO
- Economics of pre-treatment
- Consistency of pre-treatment
Different sources of water are:
- Mainland surface water: rivers, lakes, ponds and open reservoirs
- Seawater: open and deep intake or sea well
- Underground Water: deep bore wells and shallow wells
- Treated Wastewater: before and after tertiary treatment
Water chemistry for each category is typically distinct and so the treatment would be specific for each one.
Mainland surface water has higher turbidity, TSS, BOD, organic load and most importantly varying character due to various natural or man made reasons. This category calls for the highest degree of protection in pre-treatment stage.
Two options can be worked out with some modifications as per case to case basis:
Option I: Conventional Pre-treatment
Primary Screening for large particles removal
Gravity sedimentation with ½ to 1 hr hydraulic retention to remove readily settle able solids with 60-80% efficiency
Coagulant and polymer dosing to increase solids removal efficiency
Clarifier to reduce higher turbidity, TSS, BOD, TOC
and heavy metals with 80% efficiency
(Pressurised filtration through filtering media beds if turbidity is below 20 NTU)
Disinfection with chlorine, UV, Ozone, hydrogen peroxide, etc., to eliminate bacterial population
Activated carbon filter to take care of organics, surfactants and colour with 80% efficiency
Antiscalant dosing to avert precipitation of foulants in elements
Anti oxidant dosing to prevent oxidants (prevalently for chlorine)
Micron filtration for fine particulates (typically 5micron)
Option II: UF in PT
Primary screening for large particles removal
Gravity sedimentation with half to one hour hydraulic retention to remove readily settleable solids with 60-80% efficiency
Coagulant and polymer dosing to increase solids removal efficiency
Clarifier to reduce higher turbidity, TSS, BOD, TOC and heavy metals with 80% efficiency or (Pressurised filtration through filtering media beds if turbidity is below 20 NTU)
Ultra filtration to take care of organics, surfactants and colour with 90% efficiency
Antiscalant dosing to avert precipitation of foulants in elements
Anti oxidant dosing to prevent oxidants (prevalently chlorine)
Micron filtration for fine particulates (typically 5 micron)
Pre-treatment for seawater is more or less same except for a few things like presence of algae, barnacles, barium, strontium, boron and possible oil pollution. Oil land algae are floating matters and not a concern if intake is deep in sea or a sea well. To overcome algae problem, open graded sand filtration beds is the best option, if land availability is not a constraint. Dissolved air flotation is another technique which has been applied successfully in algae and oil removal.
Chlorine dosing should be avoided in seawater. If boron is a concern, then alkaline dosing in the feed stream should be tried out to get desired levels in permeate and if required boron rejection membranes which can reduce up to 90% can be used. Barium and Strontium should be put in calculation and then antiscalant dosing needs to be worked out carefully. Antiscalant soft wares are quite helpful in this direction.
Deep bore wells are most safe source of water for RO membranes. TDS levels may vary place to place but Turbidity and particulates are found in traces. Colloidal iron or silica are two silent killers which may be present mainly due to hydro-geographical topology of the region.
Following scheme can be tried out for bore well pre-treatment.
Disinfection with chlorine, UV, Ozone, hydrogen peroxide, etc., to eliminate bacterial population (mainly for PT itself)
Pressurised filtration through filtering media beds
Antiscalant dosing to avert precipitation of foulants in elements
Anti-oxidant dosing to prevent oxidants (prevalently for chlorine)
Micron filtration for fine particulates (typically 5 micron)
If dissolved, Iron is considerably high (typically above 5.0 ppm), then a separate Iron removal filter should be used ahead of media filtration.
Following are some valuable propositions to think about in pre-treatment:
i) Those constituents which are lethal even in traces should be strictly taken care of in pre-treatment. Some of them are oil and grease, heavy metals, oxidants and bio-foulants.
ii) Ultra filtration would be first choice for surface waters with fluctuating character.
iii) Full advantage of instrumentation would be taken in making system more informative and fail proof if economics is not a constraint.
iv) For right dose of chemicals, jar tests should be done in lab or at site.
v) Effective schedule of analysis for source water and at different stages of treatment is must to know about fluctuations.
vi) Quality of chemicals to be used in pre-treatment is of utmost importance as sub standard chemicals may harm system by adding unwanted impurities like heavy metals.
vii) Proper data tracking is a useful tool in assessing pre-treatment accuracy and efficiency. Daily log sheets could be of great help.
viii) Metallurgy of piping and vessels would be in accordance of corrosiveness of water.
It is important to envisage areas where things could go wrong in terms of operation. This requires a lot of knowledge, skills and imagination. Sometimes, much more is put in to the system than actually required. The financial side should be given due consideration without compromising on technical aspects, when selecting scheme of pre-treatment.
Yogesh Gautam
Center for Design & Research, Pentair Incorporation
