Modular MBR settles sludge
Zenon's modular, Z-MOD MBR, consists of a suspended growth biological reactor coupled to an ultrafiltration (UF) membrane filtration system.
The UF system replaces the solids separation function of secondary clarifiers and the polishing function of the sand filters in a conventional activated sludge (CAS) systems. The systems can be used as new-build MBRs if a bioreactor is provided or they can be used to enhance an existing CAS process by allowing the effective biomass to be increased to reduce the load on the process and enhance treatment.
The latter has been undertaken at Mathry in Wales, where a Z-MOD system installed by Meica Process incorporates Zenon ZW500 membranes, which now meets a discharge consent including ammonia at 7mg/l.
The Z-MOD system overcomes the problems associated with poor settling of sludge in CAS processes. The process can be installed in municipal and industrial projects. And the application of the technology permits bioreactor operation with considerably higher MLSS concentrations (up to 15000mg/l) than CAS systems using final settlement tanks, which can only run at up to about 3,000mg/l MLSS.
Higher MLSS levels tend to increase the risk of poor settlement and, therefore, carryover of solids from the CAS system. This often results in a failed consent and can lead to costly litigation.
At these elevated MLSS levels, effective removal of both soluble and particulate biodegradable material in the waste stream is achievable using the Z-MOD MBR. The permeate produced from typical applications generally has BOD5 less than 5mg/l, TSS less than 5mg/l, turbidity less than 1 NTU and ammonia less than 1mg/l. Conversion of a CAS system to a high-efficiency MBR process can increase treatment capacity by up to 500% in the same footprint.
A process efficiency of greater than 98% reduction in BOD5 and TSS, coupled with the absolute barrier provided by the Zenon membrane, results in a final effluent that can be designed to meet the highest discharge or reuse quality standards.
Z-MOD units are a complete filtration system only and need to be used in conjunction with a suitable aeration tank. This allows the design of both the biological and hydraulic components to be optimised in accordance with site specific requirements such as low or high strength waste streams.
Z-MOD systems are economic, pre-engineered modular units composed of industry standard components incorporating advanced programmable logic controller (PLC) based process management and control. The in-built control system actively responds to problems (such as accumulation of fouling on the membranes) by having a number of automatic cleaning strategies in place.
The units are factory assembled and tested. They are designed as complete membrane filtration systems that will fit in standard ISO shipping containers. The units can process from just a few cubic meters of flow to 3,780m3/day within a single system; this range expands if multiple systems are used.
Figure 1 shows one of the smaller Z-MOD units complete with all equipment on a single skid. Larger Z-MOD units feature separate membrane tanks and process equipment skids. The modular construction of Zenon membrane cassettes allows initial installation of the smallest capacity required, with the possibility to add more membranes as demand grows in the future.
With no clarifiers or tertiary filters, and operating with a higher biomass concentration, a Z-MOD MBR system requires as little as one tenth of the space required by a CAS plant. Civil construction costs are reduced and existing structures may be retained or adapted to provide biological treatment capacity.
The UF technology used in the Z-MOD MBR is the ZeeWeed ZW500 hollow fibre membrane, with a 0.035µm (nominal) pore size that ensures a particulate-free effluent. The membrane’s composite structure guarantees exceptional strength and durability.
The ZW500 is an outside-in membrane: water flows from the outside of the membrane to the inside of the hollow fibre. As a result, the inside of the membrane comes into contact with only filtered water and the solids to be removed remain outside the membrane, where they do not cause fouling and plugging of the membrane surface.
A schematic of the Z-MOD system is shown in Figure 2. Mixed liquor from the aeration tank is pumped to the Z-MOD system where the permeate is removed. The residual mixed liquor then gravitates back to the aeration system. The membranes are immersed in the mixed liquor held within the filtration tank situated on the Z-MOD system, and are connected to the suction side of a centrifugal pump. The pump applies suction to a header connecting the modules, drawing water through the membranes. At a typical pressure drop of 7-55kPa across the membrane, the energy associated with permeate pumping is relatively small.
On a continuous basis, air is piped to the bottom of the membrane module where it emerges as a coarse bubble stream that rises vertically along the length of the membrane fibres. The air stream scours the external surface of the membranes and transfers rejected solids away from the membrane. The oxygen required for the biological process is provided by a diffused aeration system. Waste activated sludge is pumped away directly from the aeration tank.
The membranes can be installed directly into the bioreactor on a frame or, alternatively, in the case of the Z-MOD units, in stainless steel or epoxy-coated carbon steel tanks supplied by Zenon. Wastewater is circulated continuously from the bioreactor to the membrane tanks, by one or more submersible transfer pumps, before returning by gravity to the lead end of the bioreactor. With bioreactor-immersed membranes, sludge is recirculated from the membrane tank to the lead end of the bioreactor. In both cases, this recirculation moves accumulated solids away from the membranes and creates a more uniform mixed liquor.
The separation of the biological treatment stage and the solids separation stage allows for optimum conditions to be achieved within both process steps. Due to the insensitivity of the Zenon membranes to variable MLSS concentrations, the biological stage can be designed for the most appropriate loading conditions depending on final effluent quality and sludge yield requirements.
Where an existing CAS process may be both hydraulically and biologically overloaded, the hydraulic capacity may be enhanced by adding appropriate Z-MOD units and the biological capacity increased by increasing the MLSS concentration to the most suitable concentration. The reuse of existing tank capacity and aeration systems is then possible.
At smaller sites, where periods of little or no flow may occur, the advanced control system can save energy by automatically placing the Z-MOD units in standby-mode. Simultaneously, the separate fine bubble diffused air system within the bioreactor tank continues to operate ensuring that the biomass is kept viable.
Air is supplied to the biological treatment stage from standard positive displacement blowers, and distributed to the process through a grid of fine bubble diffusers. Separate blowers situated within the Z-MOD unit provide a supply of coarse bubble air scour to the membranes.
Wastewater is recirculated continuously from the bioreactor to the membrane tanks, at a typical rate of four times the average daily flow (ADF) to eight times, depending on the total nitrogen requirement. Transfer is by one or more submersible transfer pumps, return is by gravity to the lead end of the bioreactor. This recirculation moves accumulated solids away from the membranes and creates a more uniform biomass.
The process can be enhanced easily for phosphorus reduction. The addition of a metal salt, such as ferric chloride or alum, will precipitate the phosphorus. This now-suspended solid will be unable to pass through the membrane pores. Unlike a CAS system, however, where a significant volume of metal salt is required to precipitate, coagulate and flocculate the phosphorus into readily settleable particles, the Z-MOD MBR process only requires a small dose to cause formation of a “pin-floc.” This will be blocked by the fine size of the membrane pores.
In some cases, the front end of the bioreactor is designed as an anoxic zone. This is used to assist with pH control in standard systems and for denitrification in applications where extremely low levels of total nitrogen are required in the effluent stream.
Occasionally, the Zenon membranes may require thorough cleaning. While the frequency of cleaning is a site-specific factor, it is typically required every three to six months. For this type of cleaning, the membranes typically soak in a 200mg/l solution of sodium hypochlorite for about 12 hours. In certain cases, a mild acid or caustic solution may be required to dissolve crystallised salts that precipitated inside the membrane. The membrane is resistant to chlorine and other typical treatment plant oxidants, allowing for effective disinfection of the system as required.
An example of one of Zenon UK’s Z-MOD systems has been installed by Meica Process Limited, Welsh Water’s framework process, mechanical and electrical contractor. ZENON’s UK office provided the ZW500 membranes and a comprehensive design support package. Meica designed and built the ancillary equipment around the membranes and Zenon’s process and control requirements.
The project involved the upgrade of an existing CAS system to a high-efficiency Z-MOD MBR process treating six DWF at an average treated effluent discharge of 3.6m3/h. A special feature of the plant design was the requirement to treat 1 DWF up to the FFT, relating to a capacity to treat a highly variable loading in the flow range of 2.9-22.3m3/h.
Figure 3 shows two immersed membrane cassettes as part of the Z-MOD system, and the compact installation is also shown in Figure 4.
Prior to installation of the ZENON membranes, the effluent from the CAS plant had BOD5 less than 85mg/l and TSS less than 150mg/l. The upgrade now meets a discharge consent including ammonia at 7mg/l as well as BOD5 less than 32mg/l and TSS less than 57mg/l. The system has operated reliably and provides a good stable effluent quality well within discharge consent requirements.
The market for Z-MOD MBRs is expected to grow over the next few years with the increasing effect of the European integrated pollution prevention and control (IPPC) legislation on municipal and industrial discharge consents. The ability to economically upgrade CAS systems using Z-MOD technology at smaller sites like Mathry, due to small footprint and ease of installation coupled with advanced automatic control of the membrane process, will lead to enhanced effluent quality that can meet or exceed IPPC requirements.
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