Top Ten Issues - River Mouth Osmosis

Subtitle: Renewable Energy from River Mouth Osmosis

I was recently (January, 2014) invited to speak to the student chapter of AIChE at UC-Irvine in California (American Institute of Chemical Engineers, at University of California at Irvine).  The students requested I speak to them on TopTen Issues in Chemical Engineering.  I was happy to speak to them, as usual.  I have spoken to that group three times in the past 12 months.  Previous speeches discussed Peak Oil and US Energy Policy, and Practical Chemical Engineering Tips.  While the speeches are great fun, the questions and answers portion is always my favorite.  Students have some of the best questions.

Nile River delta
credit: Chris Hadfield - NASA

One of the issues in the Top Ten speech is the subject of today’s article: Renewable Energy from River Mouth Osmosis, (RMO).   RMO is not a novel idea, having been discussed over many years.  The basic idea is to generate power from the fact that river water is fresh (contains very little salt) but the ocean into which it feeds is saline.  A suitable permeable barrier placed between fresh and salt water will allow the salt water to pull fresh water through the membrane.  Water will flow through the membrane even when the salt water is under pressure, if the pressure is not too great. 

In practice, one version of a RMO system would have all or a portion of the river enter a vertical shaft or pipe, with its lower end placed at a depth in the ocean.  The river water, pulled by gravity, flows through a conventional hydro power plant located near the bottom of the shaft, with a water turbine that spins a generator.  The water exiting the turbine would flow into a chamber that is vented to the atmosphere.  The water in the chamber then flows through osmosis membranes in the floor and walls of the chamber and into the ocean.   With careful design, the flow of water through the membranes will equal the flow of river water into the vertical shaft.   Essentially, the power generated is virtually free, produces zero pollutants, and is inexhaustible.   In a world where so much debate occurs over green power, renewable power, and carbon dioxide regulations, the RMO system receives little attention.

The items of interest to chemical engineers in a RMO system include conditioning the river water so that the osmosis membranes have a long life, and the design of the membranes.   River water is not usually very clean at the river’s mouth, having acquired silt, chemicals, and debris from upstream.  Osmosis membranes are rather finicky, and must have fairly clean water.  Filthy water causes the membranes to plug, which requires cleaning or replacement. 

Therefore, chemical engineers would be required to design screening systems to remove the larger debris, a system to prevent fish from being harmed, filtering systems to remove the silt and suspended solids, and ph adjustment to meet the membrane requirements.  The RMO membranes would have water flow in reverse direction compared to the traditional reverse osmosis membranes.  This will also require engineering to optimize the membrane. 

The reason this process, RMO power, made the Top Ten list is not so much for the chemical engineering challenges, but for the large impact the technology could have on future power production.  While the RMO process would not be feasible in some rivers, especially those that empty into shallow seas, the rivers where the right conditions exist are numerous.   There may be some rivers where the economics are not favorable, such as where treating the river water to remove impurities is too expensive compared to the value of the power produced. 

Work and research are proceeding on the RMO systems, as shown in this recent publication on membrane research.  

There are variants on the process, such as a water surface system where fresh water flows through the membrane into brackish or seawater.  The seawater volume increases, which increases the pressure.  The pressurized water then flows through a hydro turbine that spins a generator.  

Environmental concerns arise where the river enters an estuary.  It is not clear how much the ecosystem would be changed due to a RMO plant. 

For now, the RMO process shows some promise as a means to generate clean, renewable energy.  It is also worth noting that many population centers are located at the mouth of a river.  This reduces or eliminates the need to invest in long-distance transmission lines from remote power plants such as windturbines or solar plants.  Examples include New York City (Hudson River), New Orleans and surrounding area (Mississippi River), Cairo and Alexandria (Nile River), Buenos Aires (Rio Plata), and Shanghai (Yangtze River).  The world’s largest river by flow, the Amazon, has a very small city near its mouth (Belem, Brazil).  That could change if abundant and low-cost electric power were produced there. 

I expect to write more in future on the Top Ten Issues facing chemical engineers. 

Roger E. Sowell, Esq.

Marina del Rey, California 

 Update:  My list of the Top Ten Issues Facing Chemical Engineers includes:

Fresh, Clean Water from Wastewater

Process Plant Scale-Up

Large Complex Process Optimization

Coal Gasification / Liquefaction

Low-Cost Manufacturing  -  Drugs

Process Safety via Artificial Intelligence

Unlimited Renewable Energy  (RMO is but one form of this)

Nullify Atomic Weapons

Low-Grade Heat Upgrade

Improved Corrosion Prevention via Coatings  -- end update, 3-30-14 -  Roger


Update 2:  For those who want to read more about the process, a US utility patent 3,906,250  from 1975 has a good description. see link.    In this patent, the process is referred to as Pressure-Retarded Osmosis.   More than 50 subsequent patents are listed.  -- end update 2 - Roger