Small Reverse Osmosis Desalination Research Articles

Energy recovery system in small reverse osmosis desalination plant: Experimental and theoretical investigations

The shortage of potable water, especially in developing countries has attracted researchers to try and overcome this problem. Desalination is one of the most effective methods used to solve this problem. Among desalination techniques, Reverse Osmosis (RO) has higher priority than thermal desalination methods, on account of its comparatively limited energy consumption. Different methods were introduced to enhance the performance of RO desalination plants. This study focused on employing an Energy Recovery System (ERS) to enhance the performance of the small RO plant for remote areas. The unit capacity was 2.4 m3/day, occupied with two double-acting cylinders with their control elements working as an ERS. An experimental setup is used to verify the effectiveness of the proposed ERS. A simulation model is designed and implemented to enable changing system parameters of the plant. The different system pressure experimental and model results were exposed and compared. The obtained results showed good agreement between experimental and simulation model values. The effect of recovery ratio on the power saving is highlighted. The ERS saved power consumption by 80%. The cost analysis of the small RO desalination plant with and without ERS was evaluated and showed a significant reduction in total cost.

Robust model predictive control for a small reverse osmosis desalination unit subject to uncertainty and actuator fault

Abstract Actuator faults are inevitable in small reverse osmosis desalination plants. They may cause energy losses and reduce the quality of the freshwater, which may endanger human life. Model predictive control (MPC) is a model-based approach widely used to control process systems such as reverse osmosis, while considering a set of constraints. In this paper, three methods of predictive model controllers are considered for the control of a multi-input multi-output (MIMO) reverse osmosis desalination system in the presence of noise, model mismatch, and actuator fault. Formulation of enhanced constrained receding horizon predictive control via bounded data uncertainties (CRHPC-BDU) are extended for linear time-invariant MIMO systems. Permeate flow rate and conductivity of the water produced are controlled by a retentate valve and a bypass valve, respectively. The simulation results show the robustness of the suggested approach in the presence of both noise and uncertainties. CRHPC-BDU has a better performance subject to systems with model uncertainty and actuator fault up to a reasonable limit. By increasing the actuator fault up to 34%, the robustness of CRHPC-BDU is further highlighted in permeate conductivity, where the fluctuations of permeate conductivity dampen sooner than in the other two controllers.

Integrated model predictive fault-tolerant control, and fault detection based on the parity space approach for a reverse osmosis desalination unit

Actuator faults are inevitable in small reverse osmosis desalination plants. It may cause energy losses and reduce the quality of the freshwater, which may endanger human life. This paper focuses on the integrated fault detection and fault-tolerant control approach. The primary motivation of this paper is to propose a novel integrated fault detection and fault-tolerant control approach. The actuator fault is estimated using the concept of parity space approach. Then the system model is updated in the fault-tolerant control block using the information of the estimated fault parameter. Moreover, the proposed approach uses the receding-horizon predictive control-bounded data uncertainties controller, which is the robust and stable variant of generalized predictive control. The remaining uncertainty caused by the model and observer is compensated by this controller. The structure of a small reverse osmosis desalination plant is deployed. In this plant, the permeate flow rate and conductivity are controlled by a retentate valve and a bypass valve, which add a small amount of inlet to the outlet. The performances of three predictive model controllers are evaluated, and a comparison is made between their computational costs, stability, and robustness. The plant is considered to be linear time-invariant and subject to model uncertainties, measurement noise, and actuator fault in the retentate valve as efficiency dropping. The results reveal the robustness of the proposed approach concerning noise and matched uncertainties as well as its accommodation to actuator fault up to 90%.

Integrated approach in eco-design strategy for small RO desalination plants powered by photovoltaic energy

Reverse osmosis (RO) plants powered by photovoltaic (PV) panels at community scale for remote places are increasing in importance. However, RO recovery rates in these seawater desalination plants are usually below 40% because of non-optimized plant configurations. Low recovery rates increase both the global costs of the plant and the volume of concentrate discharge, which is an ecological issue. This study aims to determine the RO plant configuration that enables the highest recovery rate and leads to the lowest global costs for small PV RO plants. The choice of the plant configuration includes the type, number and arrangement of the RO modules, single or double stage configuration, the area of the PV panels, the possibility of using batteries, and of using evaporation ponds for concentrate disposal. Based on an “optimization procedure” and cost estimation, the best PV RO plant configurations were found according to the production needed. A 65% recovery rate could be possible in double stage plants producing more than 5m3·d−1 and would reduce costs. The use of a battery could reduce the costs in plants producing 5m3·d−1 because the extra cost of the battery would be compensated by the possibility of using fewer RO modules.

The integration of water vane pump and hydraulic vane motor for a small desalination system

An integration of water vane pump and hydraulic vane motor was introduced in small reverse osmosis (RO) system, in which the hydraulic vane motor was used as energy recovery device. The hydraulic performance and energy consumption of one combination of pump and hydraulic motor were investigated under different operation conditions. This type of integration pump reduces energy cost and simplifies the system setup, thus it could be an alternative choice for small RO desalination system. Our results demonstrated that the improvement of volumetric efficiencies of the pump and the hydraulic motor was the main factor to increase the prototype pump performance.

Change of energy consumption through the adjustment of feed flow rate in RO membrane process

The reverse osmosis (RO) process requires a large amount of energy for pressurizing feed flow, most of which is wasted through the brine stream. To recover the brine energy and deliver it to the feed, several methods using energy recovery devices (ERDs) have recently been used. However, the way to achieve efficient RO membrane process without ERD is also required in small RO desalination systems. In this work, the change of energy consumption upon the alteration of recovery ratio through the adjustment of feed flow rate was demonstrated. A variable frequency drive (VFD) was used to regulate feed flow rate. Permeate flux, recovery ratio, and salt rejection which are the key performance parameters of reverse osmosis were compared at the different feed flow rates, operating pressures, and feed temperatures. In addition, the power consumed in both high pressure (HP) pump and booster pump was measured and the specific energy consumption was compared. The control of feed flow rate reduced the specific energy consumption by 20%. Thus the adjustment of feed flow rate was effective to some extent in energy saving at a light cost of permeate volume and product water quality.

Application of forward osmosis in pretreatment of seawater for small reverse osmosis desalination units

Recently, membrane pretreatment has been accepted as a technically and economically viable alternative to conventional pretreatment in SWRO processes. MF, UF and NF membranes were used in these pressure driven processes, with UF most commonly recommended. Amongst these, forward osmosis (FO) driven process has been used in different water treatment operations including membrane pretreatment of wastewater. In this study, the application of a FO driven membrane process in seawater pretreatment was evaluated. This application is particularly important for small autonomous RO desalination units as it eliminates the need for chemicals which are conventionally used in pretreatment steps, the concomitant need for disposal of the chemical laden waste resulting from the process, and the requirement of qualified expertise for unit operation. The performance of commercially available FO membrane cartridges (Hydration Technologies Inc.) in terms of water flux and salt flux was evaluated using tap water and seawater as...

An experimental comparative study of the technical and economic performance of a small reverse osmosis desalination system equipped with an hydraulic energy recovery unit

An experimental study is presented of a small-scale conventional seawater reverse osmosis (SWRO) system with potable water production of 1.7 m 3/d, and the modifications made to this system by installing an hydraulic energy recovery unit and completely changing the pumps. The measured specific energy consumption of the proposed system was experimentally found to be 3.3 kWh/m 3, which is a very promising value compared to the value of 20 kWh/m 3 of the conventional system. Lowering the specific energy consumption results in the reduction of the total system's installed power, which achieves lower permeate production costs. The aim of the modifications was to test, investigate and construct a SWRO system equipped with an hydraulic energy recovery unit operating under variable conditions of pressure and flow to meet the variable power supply systems for future connection with renewable energy technologies such as photovoltaics and wind turbines. The only electrical load of the system is a brushless DC motor of 510 W maximum power, coupled directly to a positive displacement rotary vane pump with a flow rate of 900 L/h. The expected permeate production cost (excluding the energy production system cost) was calculated to be 3.68 €/m 3, while the reported cost of transported water to some of the Greek Islands is 7 €/m 3.

Desalination of brackish groundwater in Zahedan city in Iran

Zahedan city lies in one of the most arid areas in southeastern Iran. As the capital of Sistan and Baluchestan province it has grown and urbanized rapidly with a population of over 600,000. All water consumption needs are met by groundwater. In recent years, groundwater has been abstracted from the aquifer at a faster rate than the recharge because of extremely low rainfall, rapid development of the city and high population growth rate, resulting in gradual depletion of storage. The result is a continuous decline in the groundwater levels and deterioration in quality. Consequently, Zahedan is the only city in Iran with two separate pipeline systems for drinking and household requirements due to the increasing salinity of the groundwater aquifer. About 50% of fresh water is imported by tanker from Ladiz village about 100 km southeast of the city and then pumped into the pipeline system that provides drinking water at 13 public stations. Shortage of drinking water has caused interest in developing non-conventional options, including desalination of brackish groundwater and a water import project. From the beginning of 2003, three small reverse osmosis desalination units have gone into operation for desalination of brackish groundwater to provide a part of the drinking water requirements.

Technical and economic analysis for the integration of small reverse osmosis desalination plants into MAST gas turbine cycles for power generation

A technical and economic analysis concerning the integration of small reverse osmosis (RO) desalination plants into mixed air steam turbine (MAST) technologies for power generation was carried out. The simulation tool used is the computer aid reverse osmosis calculations optimization algorithm. This user-friendly software takes into account the capital cost, fuel cost and operation and maintenance requirements of each candidate RO desalination plan scheme and calculates the least-cost configuration. The results indicate that the integration of a RO desalination plant into MAST gas turbines has a minor effect on the final operating cost of the power plant.