Preparing Wastewater Effluent for Membranes

Ethanol is a love hate issue in America today. Grain farmers love it while poultry and livestock ranchers hate it; American isolationists love it while globalists hate it, and it goes on and on. Regardless of one stand on the subject, ethanol plants are in operation all over the United States. Today, 143 biorefineries are producing 8.2 billion gallons of ethanol per year (bgy). Presently, 57 new facilities are under construction with seven major expansions of existing facilities that will soon add another 5.2bgy.

Infrastructure is a big issue with local ethanol plants. Hundreds of big trucks go in and out taking their toll on local roads and bridges. Noise becomes a nuisance to surrounding neighbours. Electrical services must often be upgraded. About 2-3 gallons of water are needed for each gallon of ethanol produced. Where is this “extra” water going to come from?

Facility

The 1700 people in the rural community are contributing to renewable energy in a big way by producing 110 million gallons of fuel ethanol per year from locally grown corn. This facility is state-of-the-art for converting corn grain to fuel grade ethanol. Water usage at this location is 600 gallons per minute (gpm) 20 hours per day. This facility has two sources of water available, both being six miles away. The first is water supply reservoir that provides water to the community and surrounding area.

The second is effluent from the local municipal wastewater treatment plant, also stored in a reservoir.Both sources needed onsite treatment for uses in the production of ethanol. Hollow fiber ultra filtration (UF) membrane systems were chosen for this task. Water at the rate of 400gpm was to be drawn from the fresh water reservoir and feed one of the UF membrane systems. Part of the permeate from this system would feed a reverse osmosis (RO) membrane system for further treatment for boiler feed water and other critical uses while the remaining permeate went to cooling tower make up and other less critical uses. Another 200gpm would be drawn from effluent reservoir and supplied to a second UF membrane system also for cooling tower, wash-down stations, and other miscellaneous.

Initial Problem

It was quickly realised that neither waste water source could be fed directly to the UF membranes without some form of pretreatment. The fresh water reservoir supported a teaming population of fish, turtles, insects and some vegetation. Pumping from this reservoir six miles away entrained all of these critters and more into the supply water. The effluent reservoir, though recently treated to safe standards at the wastewater treatment plant and further treated by more filtration and disinfection was still in a reservoir standing as an open body of water. Being more nutrient rich, algae and other phytoplankton grew quickly. Some form of additional treatment was necessary at the ethanol plant to remove these solids before introducing the water to the UF membrane.

Solution

After weighing many options, the membrane supplier decided to use automatic self-cleaning screen filters for pretreatment. As shown in Fig.1, two filters were mounted in parallel on a manifold and used to treat the water from the freshwater reservoir. A single filter of same model, for commodity of parts, was used on the effluent reuse line. These filters have to be reliable, space-saving and water efficient . With a footprint of just over two square feet each, these particular new models use only about one-third the number of gallons of water during their cleaning cycles, compared to most other self-cleaning filters in the market. And most pleasing to the membrane supplier was the fact that these filters were available straight from the warehouse shelves.

Design

It was determined at the beginning of the design process that the two water sources would be kept on separate systems throughout the treatment process. The UF membrane supplier specified a filtration degree of 400 microns (40 meshes) on the pretreatment systems. Water from the freshwater reservoir was to be pumped through a six mile pipeline at 400gpm 40-60psi. Two filters of medium size are recommended for this system to assure abundant screen area and minimum rinse cycles. Even though one filter could have sufficed, it would have rinsed quite frequently. By adding two filters to the system, the screen area was doubled which decreases the frequency of rinse cycles by a factor of four since the differential pressure across the screen increases with the square of water velocity going through the openings in the screen. This also added 100% redundancy for the freshwater line since it was to be the primary water source to the facility. Reuse effluent was to be delivered to the facility at 200gpm and 40-60psi through another six mile 6” pipe.

While designing automatic screen filters, pipe size is of secondary importance. Because a screen is essentially a two-dimensional object and the filter cake that builds up is very thin before differential pressure becomes excessive, flux becomes the most critical design parameter. Flux is a design flow rate per unit area of screen surface (flux=gpm/in(whole square).

The exponential relationship of differential pressure to velocity through the screen element gives rise to very sudden and large pressure drops in the system once this differential pressure reaches a value of about 7psi. Solid characteristics like shape, plasticity, durability make a big difference in the allowable flux a system can accommodate. Add the particle characteristics to flow rate, pressure, particle size distribution, filtration degree and screen open area and there are many variables to consider in the design process. Sound hydraulics and other physical phenomenon must be adhered to but, without the qualification of “field experience”, one can only approximate a real solution to the filtration design.

Operation

The operation of the filters starts with dirty water entering the inlet (1) shown in figure 2 where it goes into the centre of the fine screen. (2) The water then passes through the line screen from the inside out & exits the outlet. (3) The unwanted solids accumulate on the inner surface of fine screen, creating a pressure differential across the screen. Once this pressure differential reaches a pre-set level (usually 7psi) a rinse cycle is activated by the factory supplied control system by opening the rinse valve (4) to an atmospheric drain. As a result pressure drops in the rinse chamber (5)and dirt collector assembly.(6) The pressure drop creates a back flush stream through the screen at the nozzle openings which are very close to the inside surface of the screen. This low pressure area sucks the dirt of the screen in a “dime” size area, similar to a vacuum cleaner. The backwash water is carried through the dirt collector ejected out of the holes in the reaction turbine. The water being ejected out of the reaction turbine causes the collector to rotate, similar to the turbine that rotates generators at Hoover dam. In addition pressure released from the hydraulic piston causes the collector assembly to slowly move upward. This combination of rotational and linear movements ensure that the entire screen area is cleaned each cycle. The cleaning cycle of each filter takes less than 14 seconds to complete. The two filters on a common manifold form the fresh water reservoir line and operates as a system. When a 7psi differential is sensed across the inlet & outlet pipes of the filters, the electronic controller will rinse filter#1 then when it has completed its short rinse cycles, filter #2 will be rinsed. During the rinse cycles neither filter is off line and therefore both filters are always providing filtered water down stream.

There is only one moving part (dirt collector /reaction turbine/hydraulic piston assembly) and then it only operates for about 14 seconds at a time so maintenance is quite simple. Annually, the manufacturer recommends that the filter be opened, the screen removed and inspected. Failure to maintain a clean screen automatically usually indicates either low pressure or a blockage in the rinse line going to a drain.

Start-Up

When the pumps were first started, the filters for both sources quickly registered a 7 psi pressure drop but were unable to clean themselves regardless of how many rise cycles were completed. The only thing to do at this point was to open the filters as see if anything could be observed that was hindering the self-cleaning process. When opened, the reason becomes obvious but the solution was not. The entire inside volume of the cylindrical screen elements were packed with tree leaves and twigs. They were so tight that the reaction turbine could not rotate the dirt collector. The only remedy to try was to fully open the supply lines before the filters and see if the debris was just what had lain the new six mile pipelines from construction operations or whether it was being drawn into the pump at the reservoirs.

The ethanol plant is drawing up to one-third of its needed water from reusable effluent. Appropriate technologies are being used to maintain the quality of water for boilers, cooling towers, wash water and other onsite water requirements. Screen and membrane technologies combine to provide the exact quality of water necessary to keep production flowing regardless of raw water sources.

Dr Marcus N Allhands (PE),

Orival Inc., USA

Contributed by Gopani Product Systems