Understanding Desalination

Understanding Desalination


Pre-treatment Filtration

Reverse osmosis membranes act as very effective filters. Therefore any suspended solids, colloids or heavy metals entrained within the water run the risk of fouling the membrane surface. An effective pre-treament system to remove these impurities before the water gets to the membranes is essential. The pre-treatment system will be designed to take into account the feed water and water quality needs.

Multi-media filtration (MMF)

This level of filtration removes course particles in the 20-25 micron range. It is suitable for a variety of intake waters. These filters use multi-layers of different media including various coarseness of sands, pumice, anthrice or garnet as filters.

Microfiltration (MF)

Microfiltration is designed to remove particles in the 0.1 - 1 micron range. The microfiltration process removes contaminants through a microporous membrane. These membranes affectively remove pathogens and large bacteria.

Ultrafiltration (UF)

Ultrafiltration is another membrane filtration process not fundamentally different from microfiltration except in terms of the size of molecules it retains. The pores of ultrafiltration membranes can remove particles of 0.001 - 0.01microns from fluids. This removes most viruses, bacteria, colloids and silts (SDI) from the intake water.

Reverse Osmosis

Desalination is the removal of dissolved salts from a solution. This can be achieved in a number of ways, with the most common forms falling into three main categories - membrane, chemical or thermal processes. Reverse Osmosis (RO) is a process that uses a pump and a semi-permeable membrane to cause the separation of dissolved salts from a liquid. The pump provides the driving force to the membrane that is greater than the osmotic pressure of the liquid. The semi-permeable membrane allows water and some ions to pass, but retains most of the dissolved salts. The reverse osmosis membrane also removes more than 98% of residual biological and colloidal matter from the feed water, resulting in a highly purified product stream. Osmosis is the natural process which occurs when water spontaneously flows from a purer solution, through a semi-permeable membrane into a more concentrated solution. Osmotic pressure is the pressure associated with osmosis. Reverse Osmosis is used for desalting sea water (SWRO) and brackish water (BWRO).

Membrane Bioreactor (MBR)

The term MBR describes the two stages of treatment that includes an activated sludge bioreactor to process BOD and nutrients and then a membrane barrier to separate solids from water. Membranes can be provided in the MF and UF pore sizes depending on the reuse application. MBR is suitable when a high level of treatment is required for reuse water, for example where high risk of human contact exists and therefore high levels of downstream disinfection is required.

Moving Bed Bioreactor (MBBR)

The term MBBR describes treatment that includes a bioreactor that contains loose plastic media to which bacteria attach and grow. This bacteria uses BOD and nutrients and reduces their concentration in water. The MBBR is followed by a clarifier to separate solids from water. MBR is suitable when a medium level of treatment is required for reuse water, for example where moderate risk of human contact exists. The MBBR process is more compact than activated sludge processes of the same capacity.

osmofloDesalination Answers:

Understanding the quality of my water

The key to efficient water treatment is to understand the quality of the raw feed water that requires treatment. Water isn’t just water - it can differ greatly from one location to another. Salt and mineral content plus the level and types of contaminants can all vary dramatically. So the starting point for any plant design is a complete and accurate water analysis. Understanding raw water quality is the key our designers need to unlock the right, most cost effective solution that can be deal with a specific client's raw water quality and meet project requirements.

What is reverse osmosis desalination?

Desalination is the removal of salts from water. This is done by either reverse osmosis or flash distillation. Reverse osmosis (RO) involves the passing of feed water through a membrane under pressure. The salts are unable to pass through and are collected in a concentrated form. Treated water is known as permeate. The process can be repeated one or more times to further concentrate the brine and reduce the percentage of rejected concentrate water. In most applications reverse osmosis is the most efficient way of desalinating water. Flash distillation involves boiling off feed water and then collecting and condensing the steam which has left all impurities and salts behind. This process is highly energy intensive and is generally only used in applications where waste steam is available from industrial processes.

Getting the best efficiency

Efficiency in overall performance begins with the right design from the start. In turn that means developing a full understanding of each client's needs taking in not just water quality, demand and ultimate use of the process water, but also how our equipment will integrate into systems already in place. Osmoflo's design engineers have the experience to know which parameters need to be considered and work closely with individual clients to develop the best design and the most suitable whole of life operations and maintenance regimes for each particular application, including the suitability of remote operation.

Energy efficiency in desalination

There is general belief that reverse osmosis desalination is energy intensive. In reality, efficiency has improved substantially in recent years. Current generation membranes operate at far less pressure than previously and that in itself significantly reduces energy consumption. Simplicity in design and the right operations regime can further reduce energy consumption. Efficient design also includes the incorporation of energy recovery devices, such as turbines and pressure exchangers - common features on many Osmoflo plants. For large plants whose purpose is to service urban populations, the requirement to integrate green energy is often mandated during the early planning phase.

Achieving high recovery of water

The key to achieving high recovery is efficient design, including incorporating the right pre-treatment technologies to properly condition feed water prior to feeding into the membranes. Experienced designers will recommend what pre-treatment techniques are required after consideration of raw (feed) water quality and application requirements. Many modern plants have a two-pass reverse osmosis process in which reject water from the first pass is further treated in a second stage to produce a brine concentrate that reduces the amount of reject water discarded in the desalination process.

The real cost of desalinated water

The actual cost of desalinated water depends on a number of factors including raw and treated water quality and the final application. It is however a lot less expensive than most people believe. For example, desalinating a litre of sea water costs about the same in energy terms as the cost of electricity required to run a kitchen appliance such as a bar fridge, for an hour! The development of new long-life membranes which also require less chemical cleaning has also substantially reduced whole of life capital costs as does efficient operation.