Clark C.K. Liu

Experiments of a prototype wind-driven reverse osmosis desalination system with feedback control

Abstract A prototype wind-powered reverse osmosis desalination system was constructed and tested on Coconut Island off the northern coast of Oahu, Hawaii, for brackish water desalination. The system has four major subsystems: a multi-vaned windmill/pump, a flow/pressure stabilizer, a reverse osmosis module, and a control mechanism. The feedback control mechanism, developed by this study, allowed this prototype system to be operated satisfactorily under mild ambient wind of 5 m/s or less. No auxiliary power source was needed. The system operational data showed that at an average wind speed of 5 m/s, brackish feedwater at a total dissolved solids concentration of 3000 mg/1 and at a flow rate of 13 1/min could be processed by this system. The average rejection rate of this prototype system was 97% and the average recovery ratio was 20%. The energy efficiency of the system was measured at 3 5%, which is comparable to the typical energy efficiency of well-operated multi-vaned windmills. Generally, the systems energy efficiency decreases as wind speed increases.

Technologies for Efficient Use of Irrigation Water and Energy in China

Abstract While the shortage of water and energy is a well-recognized worldwide natural resources issue, little attention has been given to irrigation energy efficiency. In this paper, we examine the potential energy savings that can be achieved by implementing improved irrigation technologies in China. The use of improved irrigation management measures such as a flow meter, irrigation scheduling, and/or regular maintenance and upgrades, typically reduces the amount of water pumped over the course of a growing season. The total energy saved by applying these improved measures could reach 20%, as compared with traditional irrigation methods. Two methods of irrigation water conveyance by traditional earth canal and low pressure pipeline irrigation (LPPI) were also evaluated. Our study indicated that LPPI could save 6.48×109 kWh yr−1 when applied to 11 Chinese provinces. Also, the CO2 emission was reduced by 6.72 metric tons per year. Among these 11 surveyed provinces, the energy saving potential for two provinces, Hebei and Shandong, could reach 1.45×109 kWh yr−1. Using LPPI, potential energy saved and CO2 emissions reduced in the other 20 Chinese provinces were estimated at about 2.97×109 kWh yr−1 and 2.69 metric tons per year, respectively. The energy saving potential for Heilongjiang, a major agriculture province, could reach 1.77×109 kWh yr−1, which is the largest in all provinces. If LPPI is applied to the entire country, average annual energy saving of more than 9 billion kWh and average annual CO2 emission reduction of more than 9.0 metric tons could be realized. Rice is one of the largest users of the worlds fresh water resources. Compared with continuous flooding irrigation, intermittent irrigation (ITI) can improve yield and water-use efficiency in paddy fields. The total increments of net output energy and yield by ITI in paddy fields across China could reach 2.5×1016 calories and 107 tons, respectively. So far only a small part of agricultural land in China has adopted water and energy saving technologies. Therefore, potential water and energy savings in China by adapting improved irrigation technology could be significant and should be carefully studied and applied.

Fluid flow and solute transport processes in unsaturated heterogeneous soils: Preliminary numerical experiments

Abstract Preliminary numerical experiments are conducted to investigate scale-dependent macrodispersivity, relative to solute transport, in unsaturated heterogeneous porous media. Synthetic heterogeneous soil columns are generated with the turning-bands approach. Fluid flow and solute transport are simulated with a three-dimensional finite-element model. Our numerical experiments confirm a three-phase development of macrodispersivity as a plume of contaminant moves through a heterogeneous soil. The initial phase grows linearly, then becomes nonlinear, and eventually reaches an asymptotic constant value. We find that the development of macrodispersivity in an unsaturated heterogeneous soil column can be predicted successfully by Dagans formulas which were originally developed for heterogeneous aquifers. For the study of contaminant transport in unsaturated soils, the travel distance one would normally deal with is much shorter than that required to reach asymptotic macrodispersivity; thus, the two pre-asymptotic phases should be the principal concern in modeling contaminant transport in unsaturated soils. The first phase of the macrodispersivity development corresponds to what is normally referred to as the advection period during which no significant lateral mixing takes place. The linear system model for contaminant transport, which uses the travel time probability density function as the models impulse response function, is valid only for the first phase and should therefore be applied only to the simulation of near-field contaminant transport within the advection period.

Solute transport modeling in heterogeneous soils: Conjunctive application of physically based and system approaches

Abstract One of the most acute environmental problems is groundwater contamination caused by residuals of agricultural pesticides in soils. Mathematical models that predict the contamination potential of chemical inputs on the soil surface or in the upper soil zone are increasingly used as an important management tool. Mathematical models can be formulated by following physically based and system approaches. In this study, models based on these approaches were developed to simulate chloride transport in field soils. Given a common data base, concentration profiles calculated by these models showed a rather close similarity and agreement with measured profiles. The conjunctive use of these modeling approaches indicates that the dispersion coefficient in the physically based convection-dispersion model is not a constant; instead, it changes with the longitudinal travel distance. As a result, when these models are used to predict solute transport into deeper soil, the physically based model with a constant dispersion coefficient tends to underestimate solute spreading. This is not a problem of a linear system model whose impulse response function excludes an explicit dispersion term.

The Development of a Renewable-Energy-Driven Reverse Osmosis System for Water Desalination and Aquaculture Production

Water and energy are closely linked natural resources - the transportation, treatment, and distribution of water depends on low-cost energy; while power generation requires large volumes of water. Seawater desalination is a mature technology for increasing freshwater supply, but it is essentially a trade of energy for freshwater and is not a viable solution for regions where both water and energy are in short supply. This paper discusses the development and application of a renewable-energy-driven reverse osmosis (RO) system for water desalination and the treatment and reuse of aquaculture wastewater. The system consists of (1) a wind-driven pumping subsystem, (2) a pressure-driven RO membrane desalination subsystem, and (3) a solar-driven feedback control module. The results of the pilot experiments indicated that the system, operated under wind speeds of 3 m s-1 or higher, can be used for brackish water desalination by reducing the salinity of feedwater with total dissolved solids (TDS) of over 3 000 mg L-1 to product water or permeate with a TDS of 200 mg L-1 or less. Results of the pilot experiments also indicated that the system can remove up to 97% of the nitrogenous wastes from the fish pond effluent and can recover and reuse up to 56% of the freshwater supply for fish pond operation.

Simulation of 1,3-dichloropropene in topsoil with pseudo first-order kinetics☆

For fast-degraded chemicals such as 1,3-dichloropropane (1,3-D), their long persistent time in topsoils cannot be explained by the ordinary first-order kinetics of biodegradation that is commonly used in the simulation of chemical transport in soils. The Monod kinetics of biodegradation, which is usually defined as the mathematical relationship between the residual concentration of the growth-limiting substrate and the specific growth of degraders in laboratory reactors, was found to be responsible for the phenomenon of “decelerated biodegradation”. To take advantage of both the simplicity of first-order kinetics in transport modeling and the realistic description of Monod kinetics for a physical situation, a simplified method was used to represent Monod kinetics with the corresponding pseudo first-order kinetics. Pseudo first-order constants fitted with Monod kinetics were later substituted into the transport model. A satisfactory agreement between field measurement and simulated results using these constants was achieved.