Utilizing a holistic design approach for desalination leads to energy savings & lower total cost of ownership
Balancing competitive capital costs for a new desalination plant with features that will ensure longevity and reliability of operations, as well as optimized energy consumption, is a constant challenge. The gold standard for evaluating the design, construction and operation of the facility is the projected total cost of ownership (TCO) over the plant’s life.
Although seawater reverse osmosis (SWRO) increasingly has become more cost-effective and energy-efficient compared with the technology of a few years ago, energy requirements still constitute close to 60% of the operating costs of a SWRO plant. In fact, energy consumption and biofouling are by far the two biggest pain points of membrane desalination.
From an operations standpoint, biofouling is a serious and recurrent problem in SWRO plants that often leads to a reduction in productivity, either by decreased flow or increased downtime due to unanticipated cleaning and maintenance. Many times, SWRO plants are not able to sustain their original design performance parameters over a prolonged operation period of even a few years due to recurrent and unexpected biofouling. This leads to reduced reliability and increased power consumption due to the resulting increases in the differential pressure.
Lower Energy & Higher Reliability
Aquatech recently had to focus on the goal of lowest TCO for its desalination plant in Visakhapatnam, India, where it entered into an agreement with Hinduja National Power to supply 12.5 million liters per day of desalinated water on a build-own-operate-transfer basis over 25 years. The company utilized a design approach it refers to as LoWatt, a process aimed at addressing energy consumption and biofouling for an overall system with higher reliability and lower energy consumption.
All desalination systems today incorporate some type of energy recovery device to optimize energy consumption. To achieve sustainable lower energy consumption, it is important to ensure that the membranes do not foul and that the differential pressure is constant. It also is important to have some kind of method available to clean membranes in the initial phase of biofouling formation before it impacts differential pressure and before any fouling becomes permanent and starts to impact plant performance in terms of water production, power consumption and product quality.
The process used at the Hinduja Plant included a few steps that, when combined, result in lower energy consumption and reduced biofouling. This process design was derived from significant research and applied testing done by Aquatech over the past several years.
The following information discusses the reasons for the various steps in the process. It also reports data gathered during a performance review of the membrane plant, first without the various steps and then using the complete LoWatt process.
Pretreatment Through UF
Pretreatment is done with ultrafiltration (UF) membranes, which give more than a 6-log reduction of bacteria and 1- to 2-log reduction of viruses. The UF permeate provides a silt density index (SDI) of less than 3 and often between 1 and 2. The UF is able to remove the majority of the suspended particles, including those that are colloidal in nature. It also removes some biofoulants, but UF alone is not able to remove all of the organic contaminants that cause fouling on membranes.
Biofoulant Removal Filter
Further treatment happens through a biofoulant removal step by removing the majority of nutrients such as humic acids, polysaccharides, proteins, amino acids, carbohydrates, bacteria, viruses and other potential contaminants that aid biofouling. Because the pre-filtered water first goes through the UF, the biofoulants removal filter delivers large quantities of treated water with much lower turbidities and SDI while removing the majority of potential contaminants that can cause fouling. The typical SDI value at the outlet of this filter is less than 1 and typically close to 0.6 to 0.8. The process highlights the importance of biofoulants at the downstream of UF, which is critical for eliminating or minimizing biofilm formation at reduced flux of reverse osmosus (RO).
Reduce the Flux
The system design and plant operation is done at lower flux of around 6 to 8 gal per sq ft per day based on feedwater quality, permeate quality requirement and temperature range. Operating at a lower flux reduces the concentration of bacteria and nutrients over the membrane surface and minimizes the buildup differential pressure. Moreover, at this reduced flux, operating pressures are lowered significantly by at least 10% to 20%.
The biggest benefit at this level of flux, however, is that the biofilm formation is reduced to insignificant levels, especially when it is pretreated with UF and the biofoulants removal filter mentioned above. This ensures that the energy consumption design is optimized to start with, and remains low on a sustained basis due to reduced or insignificant biofouling. Over a period of a day’s operation, the increase in differential pressure is less than 0.1 per square centimeter and, more often, is lower than any detection limits. In addition, due to reduced driving pressure across the membrane, whatever fouling happens is not firmly attached to the membrane surface and therefore can be easily removed under mild cleaning conditions.
With the precautions in pretreatment as described, the residual foulants are not able to adhere to the membrane surface.
Unique Cleaning Regimen
To further augment the process of a cleaning mechanism to overcome any biofouling before it starts, the Hinduja plant uses a unique methodology based on the natural osmotic pressure differential between the reject and permeate water.
When the system is stopped with a continued regulated flow in the feed side, which allows the reject water to remain in the feed side, there is a steady flow of water from the permeate side to the feed side. The permeate flow continues to the feed side due to concentration differential, which is maintained by makeup reject water flow through a clean-in-place system. When this process continues for 10 to 15 minutes, any biofilm is dislodged from the membrane surface.
Because the plant has been designed at lower flux and the feedwater has been filtered through UF and passed though a biofoulant removal filter, the buildup of any biofilm can be cleaned and the pressure drop is reduced.
This process does not use any cleaning chemicals on a daily basis, but rather the brine generated in the reject of SWRO plants.
The experience at the Hinduja plant was consistent with pilot projects used to validate the LoWatt process. The results indicated that:
- The differential pressure drop across the membrane did not increase and remained constant.
- The RO system with the LoWatt process did not require any cleaning. The advantages of LoWatt over conventional RO were differentiated in a pilot project where membranes in the conventional plant were cleaned seven times during same duration and similar conditions.
- No changes or increase in power consumption were found with LoWatt process.
The experience at the Hinduja plant and in pilot studies demonstrated that the LoWatt process represents a development in bringing down the cost of desalinated water by as much as 25%.
Besides realizing lower initial power consumption, the system also is able to sustain the design power consumption parameter without compromising product quality, thereby providing a predictable lifecycle cost for a desalination system. This means more system availability and reliable operations, without expensive and time-consuming chemical cleaning regimes—all resulting in lower TCO.