MBR technology is well-established in industrial wastewater applications and it is currently making large inroads into municipal applications too. Here three plants in widely differing locations are discussed, although all are sited in rural areas: Bega Valley Sewerage Programme (BWSP) in Australia, Leidingen Ihn in Germany and Mar de Ontigola in Spain. The reasons for choosing MBR technology and the particular way in which this is implemented in each case is closely related to the environmental conditions of the areas in which they are sited, and the requirements of the local authority and the residents.
In an MBR, ultrafiltration membrane modules are submerged in the activated sludge combining the biological step and the solid-liquid separation into a single process. Since the membrane acts as a barrier, this improves the effluent quality. Also, the membrane barrier eliminates the secondary clarifier and allows the activated sludge to be more concentrated. This reduces the volume requirement for biological tanks, which saves on space and construction costs. Overall, the MBR process reduces footprint significantly compared to the combination of wastewater treatment followed by sand filtration or ultrafiltration. The footprint savings due to the wastewater treatment plant alone can be as much as 50%, and there are additional footprint savings since the additional tertiary filtration steps are eliminated.
There are several reasons why MBR technology can be an appropriate choice for rural areas. MBR systems provide excellent water quality without the risk of upsets that can occur with a secondary clarifier. An MBR system can operate with minimal supervision but still provide water of a quality appropriate for reuse. Many MBR systems in remote areas are designed with a SCADA system allowing remote access to the data. An operator visiting the plant on a daily or weekly basis to perform checks can do routine maintenance and top-up chemical tanks. MBR systems are modular and can be designed to easily accommodate additional future demand. MBR systems take up approximately half the footprint of a more conventional wastewater treatment plant making them less visible to the community.
Many rural areas derive income from tourism and MBR technology helps by providing high quality water from a system that is less environmentally intrusive. Some communities have selected MBR technology to provide water for discharge in order to maintain the quality of their waterways. MBR effluent can be recycled to provide additional water to the community so that sufficient water resources are available to cope with the requirements of the tourist season.
MBR effluent is reused today for irrigation of golf courses and parks and for toilet flushing in hotels. The village of Cloudcroft in New Mexico is constructing an MBR system that will be part of an indirect potable reuse scheme to provide additional potable water to the village, which has 750 permanent residents and a transient population that can exceed 2,000 people at the weekends and during the summer tourist season.
There are many different configurations of MBR technology but one example that optimises both membrane and module design is the PURON® submerged hollow fibre UF module from Koch Membrane Systems. The patented module is designed to avoid the clogging and sludging that is an issue with some MBR module designs offered today. The module features hollow fibre membranes with a pore size of approximately 0.05 micron. The lower ends of the membrane fibres are fixed in a header while the upper ends are individually sealed and are free to move laterally as shown in Figure 1. Solids and particulates remain on the outside of the fibres while permeate is sucked out of the inside of the fibres by means of a vacuum.
The fibres are arranged in bundles and are submerged vertically into the activated sludge. To maintain the filtration rate of the membrane modules, scouring is carried out at regular intervals using an air nozzle integrated into the centre of the bundles. The central arrangement of the nozzles inside the membrane bundles reduces the energy consumption, because the air is injected at the place where the risk of sludging is highest. The module design is such that even hairs and fibrous compounds are removed reliably from the system, so that a coarse prescreen can be used, thus improving capital and operating costs. A special feature of these membranes is their enormous mechanical strength provided by a braid inside the membrane material. The individual fibre bundles are connected in rows. Several of the rows are mounted into a frame made of stainless steel to form a module as shown in Figure 1. The free moving fibres combined with central aeration provide stable filtration during plant operation, long membrane life, and low operating costs by reducing the need for energy, cleaning and maintenance.
Bega Valley is located on the south eastern coastline of New South Wales (NSW) and includes the towns of Cobargo, Wolumla, Kalaru and Candelo. The capacity of the valley’s wastewater treatment systems was being stretched by urban growth and by seasonal population increases during the holiday period. Some unsewered villages in the valley were at risk from environmental and public health issues caused by discharge from septic tanks. In order to maintain compliance with Australia’s Environment Protection Authority regulations and to enhance environmental outcomes, the Bega Valley Shire Council developed the Bega Valley Sewerage Program, which includes the installation of new pressurized sewage collection systems coupled with membrane bioreactor treatment plants in the towns of Cobargo, Wolumla, Kalaru and Candelo. The Program was carried out by an Alliance between Bega Valley Shire Council and a private sector company, Tenix Alliance.
The programme, completed in 2007, also includes the operation and maintenance of the new and existing sewage treatment plants for ten years, with an option for Council to extend the agreement with Tenix Alliance for a further five years. All collection systems will remain the responsibility of Council. The intention is to keep capital and operating costs at a minimum and to produce a very high quality effluent for reuse. Reclaimed water from the MBR plants will be used in an irrigation scheme on public facilities such as the Cobargo Showground, Wolumla Recreation Reserve, Candelo Showground and the Sapphire Coast Turf Club, replacing potable water as the primary source. In the future, reclaimed water may also be used for toilet flushing and to provide a vehicle wash down facility.
The communities themselves each have specific requirements which the sewerage programme was to accommodate:
• Cobargo is a rural residential settlement in the north of the Bega Valley Shire characterised by foothills and valleys. Surrounding agricultural land is dominated by dairy and beef grazing. The village has a permanent population of around 400 people and also has a holiday population during peak tourist periods. The village has a busy commercial area located on both sides of the highway. Cobargo previously used septic systems consisting of absorption trenches, transpiration beds and aerated systems that were considered to be ineffective for the number of households serviced in the area and the soil type.
• Wolumla is a small rural village to the south of Bega Valley, whose economy relies on beef, dairy and sheep grazing. The township has a permanent population of around 300 people and also has a small holiday population during peak tourist periods. The township is predominantly low density urban housing with a small commercial area located on the main street including a hotel and general store. Like Cobargo, Wolumla also previously relied on septic systems.
• Candelo is approximately 20 km southwest of Bega Valley and has a permanent population of about 350 people and a small commercial area located on the main street. Dairy and beef farms along the Candelo Creek floodplain extend into the rolling hills surrounding the village. Candelo also relied on septic systems.
• Kalaru is a small rural residential settlement located in the Bega Valley Shire on the far south coast of New South Wales. The township has a permanent population of about 150 people with a considerable increase in population during peak holiday periods. The township consists of mainly low density housing and contains a large caravan park which caters to both holiday makers and permanent residents.
The surrounding landscape consists of fairly flat topography on a floodplain and a number of swamps and wetlands.
The four MBR systems are identical, resulting in significant cost savings for the Bega Valley Shire Council and the community. Each system is designed for 800 people equivalents or an annual average flow of 180 m3/day (47,000 gpd). The maximum daily flow is 300m3/day (80,000 gpd) and the peak hourly flow is 360 m3/day (95,000 gpd). Each plant includes prescreening with 3mm perforated plate screens followed by a biological treatment system with nitrification and denitrification. The UF membranes are submerged in the mixed liquor and used to separate the treated wastewater from the suspended solids. Each plant has a footprint of approximately 20 x 15 meters. Each system includes 2 modules containing 235 m2 of membrane. The membrane modules are type PSH 500 C2-8 and were supplied by Koch Membrane Systems.
The Bega Valley Sewerage Program received a “High Commendation” in the Engineering for Regional Communities category of the 2006 Engineers Australia Sydney Division Excellence Awards 2006.
Leidingen and Ihn are two rural settlements near the French/German border. Compared to Bega Valley, the area has a much higher annual rainfall. The plant located at Ihn receives wastewater from around 700 local residents. The plant – operated by Entsorgungsverband Saar – is the first in Germany to be constructed with an aerated lagoon and an upstream membrane biological stage. It is part of the EU-Life III programme whose Life-Environment component finances innovative environmental demonstration projects with the aim of bridging the gap between research and development and large scale applications.
The mixed wastewater (wastewater and rainwater) from the locations Ihn and Leidingen is pumped through a pressure pipe to the wastewater treatment plant. When rainfall is heavy, a certain quantity of the mixed wastewater is stored in the sewage network and routed later to the treatment plant. An overflow tank for rainwater in Leidingen and a storage channel in Ihn with a comparatively low volume of 60 m3 respectively are used for this purpose. The majority of the rainwater is treated in the lagoon of the wastewater plant.
The lagoon which also operates as a buffer for rainwater provides a natural, cost-effective operating unit which reduces the volume of water routed to the upstream, highly technological stage. This plant allows operating costs to be saved in contrast to purely technical membrane wastewater treatment plants.
The values in the effluent from the plant in Ihn are far below the official limit values for the discharge of wastewater into surface water bodies and the permeate from the membrane filtration unit is so low in bacteria concentrations that it complies with the EU Bathing Water Directive.
Firstly, the wastewater is fed via an overflow for rainwater into the inlet area of the aerated lagoon where coarse solid matter such as sand, gravel and rubble sinks to the bottom. Lighter solids (oils, sanitary products), that float on the surface, are retained at an inverted weir situated between the inlet and the lagoon and removed later. The wastewater flows from the inlet into the aerated lagoon where the sludge settles at the bottom of the tank. In addition to this mechanical treatment, partial biological treatment takes place in the pre-treatment pond using bacteria that are naturally present in mixed wastewater. These bacteria reduce organic contamination in the wastewater using oxygen which is fed into the water by a surface aerator. The partially treated wastewater is then routed via a pumping station from the pond to a fine screening unit in the building to remove the smaller solids. Following this, the wastewater flows freely into the activated sludge tank.
More extensive biological treatment of the wastewater is carried out in the activated sludge tank under optimised technical conditions. This tank is divided into an aerated and a non-aerated zone where different degradation processes take place. Aerobic bacteria break down the organic carbon compounds and convert ammonium into nitrate (nitrification) in the oxygen-enriched, aerated zone. Nitrate is then degraded further in the non-aerated zone by anaerobic bacteria that split the oxygen, chemically bonded in the nitrate, under anaerobic conditions. The gaseous nitrogen formed, is then released into the air. No environmental pollution is caused by this process since nitrogen is the main component in air. The wastewater is recirculated through the tank so that these two stages for nitrogen degradation, i.e. nitrification and denitrification, are in continuous, alternative operation. After biological treatment, the water still contains the biomass (flakes of bacteria), small germs and pathogenic germs which have to be filtered out in the membrane unit.
Sited at Ontígola in the province of Toledo, the membrane bioreactor plant is being built by aqualia infraestructuras to process waste water from the local business park for use in both the agricultural irrigation scheme and the nearby Mar de Ontígola nature reserve. The Reserva del Regajal-Mar de Ontígola was granted nature reserve status in 1994 and its 635 hectares of wetland are home to a number of important species including rare butterflies that are in danger of extinction.
The new plant will process 1200 cubic metres of wastewater per day after it is commissioned in May 2008. Ontígola Business Park consists of a mixture of office and light industrial units and this scheme is part of a growing trend to increase water reuse and make the most of this limited resource in the semi-arid areas of Southern Europe. The importance of making the most of central Spain’s water is clear, given the generally held belief that traditionally dry areas are going to get drier in the future. But the investment in MBR technology is also a demonstration of the local authority’s commitment to dealing with the situation in an environmentally sensitive manner by assisting local farmers and the environment at the same time.
The effects of climate change are already evident in decreasing rainfall and increasing aridity. Membrane technology, as illustrated in two of these three examples, has an ever-more important role to play in water conservation. Quite apart from the scarcity of this important resource there is the growing requirement to recycle wastewaters in a cost-effective manner for the sake of the environment – whether it has a direct use or not. •