ULTRA FILTRATION SYSTEM

Ultra Filtration is a low pressure-driven mechanical process used for separating and concentrating suspended solids, colloids, and high molecular weight materials while allowing water and assorted chemicals to pass through. When compared to other membrane processes, an Ultra Filtration system requires a much lower operation pressure, and the backwash water is sent to the headworks of the wastewater treatment plant. The Sigma Ultra-Filtration design offers several advantages over conventional methods such as:

o Low pressure operation
o Integrated Air
o Enhanced backwash
o Minimized power and chemical consumption
o Minimal manpower attention, PLC controlled
o Eco-friendly – minimal chemical handling and solid waste production
o Easy capacity expansion by simply adding membrane modules
o Cost effective and attractive pay-back period

The Feed Pump will initially pump water through a series of polypropylene 50 micron pre-filters, which ensure the removal of debris larger than the inside diameter of a UF membrane’s hollow fiber. The UF feed water then flows through a custom-made Feed Header, which will evenly dispense water directly against an array of semi permeable back washable Ultra Filtration membranes. Permeate water is collected into a custom-made Permeate Header whereas foulants will be captured in the membrane. A flow meter is located on the Permeate Header to monitor UF permeate production. Once the UF membrane becomes “loaded” with captured material from the waste stream the membrane will need to be backwashed. The UF membranes will typically recover 90-95% of the feed water, and the remaining 5 to 10% (backwash water) maybe reduced with efficient backwash practices. Utilizing a separate Backwash Pump, the backwash process requires that pneumatic valves re-direct UF permeate through the custom-made header system backwards through the UF membrane. The backwash flow rate is generally higher (2 to 3 times) than the feed rate to facilitate hydraulic cleaning of membrane fibers. The backwash process is initiated by either of two PLC controlled methods. The first method utilizes a set of Pressure Transducers; one transducer is located on the Feed Header and the other on the Permeate Header. The Pressure Transducers monitor the pressure drop across the membranes. Once the pressure differential across the membranes reaches 20 PSI, the PLC will initiate a backwash process. The second backwash initiation method is through elapsed time. Depending on client preference, Sigma will utilize one or both of the backwash initiation processes. Backwash duration and frequency are to be determined during the commissioning stages and is site specific. All backwash water is directed into a custom-made Backwash Header and returned to the wastewater treatment facility. Every 3-4 backwashes, the UF membranes will be required to have a Chemical Enhanced Cleaning. A Chemical Backwash Sequence (CEB) maximizes membrane life and minimize down times due to system overloading. The concept of a CEB is that during specific backwash sequences, chemical pumps are interlocked to operate with the Backwash Pump to dose cleaning chemicals inline. The Chemically Enhanced Backwash (CEB) can be separated into two separate chemical addition sequences. The first and most frequent sequence doses NaOCl. The second CEB chemical, sodium hydroxide, is generally dosed once in every 24 hour period at an approximate concentration of 50 mg/l. The utilization of CEB varies between applications and may not be applicable for this system. Some additional chemical cleaning agents typically used are Hydrochloric Acid and Citric Acid. The type of cleaning chemicals is dependent upon the wastewater characteristics and will be selected during the commissioning stages to optimize system performance and operating costs. The Clean in Place (CIP) portion of the UF system will be performed once in approximately 14 days (or as needed due to fluctuations in water quality) with the CIP system. The CIP regiment shall require concentrations of citric acid, sodium hydroxide, and sodium hypochlorite to be circulated through the membranes for the duration of 6 hours. Piping allowances in the original design yield easy operation of the cleaning process with minimal manpower intervention. The UF membrane integrity should be tested periodically using a Pressure Decay Test. This short test is designed to detect and verify that the system is operating within the original design. Compressed air is engaged to pressurize the membranes for the duration of one minute. If a broken membrane fiber is detected an alarm will sound and the faulty membrane module can be taken out of service or replaced.

 

REVERSE OSMOSIS SYSTEM

As a pre-treatment method to any Reverse Osmosis (RO) system, Ultra Filtration provides the most effective and efficient protection against turbidity and suspended solids, therefore prolonging the operational life of RO membranes. Osmosis is a natural process in which a fluid passes through a semi-permeable membrane from a higher concentration to a lower concentration. Reverse osmosis (RO) is a separation process where a membrane is used to "filter" dissolved solids. High pressure on the feed side is used to "reverse" the natural osmotic process, with the semi permeable membrane permitting the passage of the water while rejecting most other dissolved contaminants. The process of Reverse Osmosis forces water with a greater concentration of contaminants, using an operating pressure ranging generally from 200 to 800 PSI, through a selective semi-permeable membrane. This produces clean water with a low concentration of contaminants, named permeate. The membranes, in general, pass water very easily because of waters small molecular size; but will prevent many other contaminants with a larger size from passing through. The specific process through which a membrane functions is called ion exclusion, in which concentrations of ions at the membrane surface form a barrier that allows water molecules to pass, while excluding other substances. Water will typically be present on both sides of the membrane, but each side having a different concentration of dissolved minerals. The effectiveness of an RO system depends on initial design, membrane selection, feed water characteristics, feed pressure, total dissolved solids concentrations, and temperature. The rate of production or “flux” will gradually decrease over time due to the slow accumulation of material within the membranes pores. The gradual flux decrease is common and permanent in all membranes. Typically membranes need replacement within 5 to 7 years depending on feed water characteristics. The RO Feed Pump will initially transfer water through a series of Cartridge pre-filters, which ensure the removal of debris large enough to prematurely foul the RO membrane. The Feed pump provides adequate flow and pressure to the RO High Pressure pump. Prior to the High Pressure pump is a series of instrumentation and chemical dosing apparatus, which ensures adequate feed water chemistry to the RO membranes. Sigma incorporates a precaution to ensure the RO membranes are protected from excess scaling; one chemical pump doses adequate amounts of anti-scalant. If oxidants such as chlorine are present in the feed water, a second chemical pump will dose SMBS. The High Pressure pump forces pressurized water through a custom-made Feed Header which will dispense water directly against an array of Reverse Osmosis Vessels. The RO Vessels house the RO Membranes and assist in channeling both concentrate and permeate streams into the proper collection headers. Sample valves are included on each RO Vessel to monitor individual Vessel production performance. Conductivity and flow meters on the permeate lines record the instantaneous measurements. The HMI provided also allows data trending. A series of stainless steel oil filled panel mounted pressure gauges monitor system pressure at:

1) RO High Pressure pump suction header
2) RO High Pressure discharge header to monitor RO vessel feed pressure
3) Pre and post cartridge pre-filters to monitor differential pressure
4)Concentrate header to maintain permeate recovery
5) Permeate header

All operation and monitoring parameters (pH/ORP, TDS, Flow, and Pressures) can be viewed in one centralized display area. Additional design and operation features on the Sigma RO are as follows:

o High Pressure Pump Discharge Valve to ensure suitable flow to membranes Variable
o Frequency Drive controls for high pressure pump operation
o Pressure transmitters, temperature transmitters, high and low pressure switch
o Auto feed valve
o Permeate divert valve
o Control valves for concentrate throttle.
o Centralized HMI and display with on screen display of operating parameters
o Permeate Throttling Valve to apply suitable permeate back pressure and flux balancing
o ORP meter or sulfite controller to monitor oxidants in first pass
o Concentrate and Permeate flow meters