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6.11 Case studies (Topic k)

6.11.1 Case study 1: real time control of urban drainage and sewerage system in Bolton, UK^*

The existing Bolton Town Centre sewer system now serves a population of 90 000 in an area of some 4 500 Ha, or approximately one third of the total Bolton population. The network is comprised largely of brick sewers which were constructed between 1870 and 1930 to take both foul and surface water. The Croal Valley/Middlebrook trunk sewer was initially constructed in the 1930s to intercept the direct discharges and thus reduce pollution of the Middlebrook and River Croal. However to minimize the size and cost of this sewer numerous overflows were retained to restrict the flows passed to the newly constructed sewer. Following development of Bolton as a town with a population rising to 250 000, river pollution of the Croal caused by the 42 crude overflows has gradually worsened and condition have deteriorated to an unacceptable level. In a study undertaken in 1987, possible solution were identified to rationalize the number of overflows in the catchment and significantly reduce pollution.

Two off-line retention tanks with a capacity of respectively 9000 and 2000 cu. m were built and completed in 1992. Two more tanks were commenced in 1992, one being a 1250 cu. m on-line oversized gravity sewer, the other a 10 000 cu. m off-line sump style tank from which a pump returns the sewage on cessation of the storm. This construction programme has been designed to alleviate flooding and pollution. The next step was to rationalize the operation of the scheme so that:

• the flow will be accommodated in the downstream sewers;
• no downstream flooding will occur;
• no spillage takes place at downstream overflows;
• the receiving treatment plants will be able to handle the additional load;
• and sewage will not stand in any of the tank for more than 24 hours, taking into account that no sewage can be passed from tank to tank.

The overall objective of the scheme was to maximize the use of the storage facilities during storm events to eliminate flooding, minimize pollution to local rivers and to optimize the use of the treatment works.

The system for controlling the operation involves the adaptation of existing telemetry equipment collecting data on a daily basis from ten raingauges located around Bolton together with sewers depth monitors, major overflows and treatment works. This links to a second system providing "alarm data" required for reactive maintenance of small pumping stations and to monitors installed at the above-mentioned tanks. Other monitors cover remote pinch points on the trunk sewer. The system collate all data at a single System Control And Data Acquisition (SCADA) Masterstation based at the receiving treatment works. The control computer interfaces with the computer to extract relevant data for simulation. Simulation and optimization of the system in real time is undertaken using MOUSE ON-LINE and control decisions sent back to the SCADA to enable activation on site.

MOUSE ON-LINE is based on a modular design with two blackboards, an external to exchange information with SCADA and an internal for the exchange of information between the modules constituting MOUSE ON-LINE. The main modules are:

• The rain forecasting module. Based on on-line rain measurements and a description of a growth and decay time profile for the rain, a rain forecast is computed.
• The runoff forecasting module. Based on the rain forecast and a description of the network topography, a forecast of the sewer load is computed.
• The control module. Based on the forecasted sewer load, a control strategy is selected and the corresponding control actions set out.

The system works in three modes, viz. monitoring, forecasting and controlling. It is based on close monitoring of the performance of the tanks, treatment works and network overflows and will provided archive data of the implications of various storms and control actions. A set of criteria specify when the mode is changed. The change from monitoring to forecasting is needed when the current situation in the sewer indicates that information about the expected situation is necessary. This could be the case when the intensity of the rain or the inflow to the treatment plant or levels in overflow structures exceed biological capacity ; in this case, the system changes back to monitoring.

^* Excerpted from: Sharman , B.J. & Tidswell, R.G., 1993

6.11.2 Case study 2: optimization of nutrient removal in the wastewater treatment plant Zürich-Werdhölzli, Switzerland^*

The plant at Werdhölzli is the largest treatment plant in Switzerland. It handles wastewater from about 500 000 pe. A second plant receiving wastewater from the city of Zürich is located at Zürich-Glatt and handles about 100 000 pe. The plant at Glatt will cease operation by the year 2001 and the wastewater it handles now will be transferred to Werdhölzli by tunnel and the plant will be upgraded. The total volume of wastewater was 225 300 m³ per day in winter of 1997 and the total chemical oxygen demand of the primary effluent was 53 500 kg per day, and, respectively, the total Kjeldahl–load 6 410 (of which 1050 was digester supernatant), total nitrogen 4 700 and phosphorous 895 kg per day. It should be noted that the COD/N ratio shifted from 7.5 to 8.5 in the last 10 years which allows a significant improvement of the denitrification capacity. Since phosphate was banned from detergents in 1986, the phosphate load did not change significantly during that period and the slight increase of the P-load might be due to polyphosphates used in dishwashers. Simultaneously, the COD-load increased substantially due to increase discharges of industrial wastewater, enzymes used in households and grease removal tanks.

In 1986, Werdhölzli was enlarged for permanent nitrification with an average total nitrogen concentration in effluent of less the 2000 mg/m³ at 10 °C. The initial activated sludge system operated as a partial two stage treatment with a fully aerated pre-step and consists of two lanes each having six parallel activated sludge tanks (5000 m³ each) and six secondary clarifiers (6000 m³ each).

Combining the flows received at the Werdhölzli and Glatt plants and, at the same time reaching high levels of nutrient removal required careful studies and the optimization of process and structural design and operation of the plant at Werdhölzli involving:

Hydraulic capacity: During the 1980s and 90s, the amount of infiltration was substantially reduced. The dry weather peak flow of both plants is now about 3 cbm per second which allows a substantial reduction of storm peak flow from 9 to 6, taking into account the 40 000 m³ stormwater tank located before the plant at Werdhölzli which reduces the overflow of untreated stormwater to less than 0.5% of the total annual wastewater flow. The existing aeration tanks can, thus handle the inflow from Glatt, even with the provision of anoxic zones (see below) and if the concentration of activated sludge is increased by some 50%. Reserve capacity is still 10%.

Installation of anoxic zones and reduction of oxygen input during primary treatment: During a pilot operation, anoxic zones of 28 vol% were installed in two parallel aeration tanks in one of the lanes at Werdhölzli, dived into two compartments with a volume of 700 m³. This reduced the nitrogen concentration in the secondary effluent by about 40% to an average of 10 g nitrate-nitrogen per cbm. Simultaneously, the energy for oxygenation was reduced by about 15%. Average effluent alkalinity increased by about 0.6 mM to above 3 mM which reduces corrosion of cement and improves nitrification. In the light of these positive results, anoxic zones were also installed in the remaining lanes. Additional measures to improve denitrification included:

• Improved sludge blanket by a reduction of scraper speed in the secondary clarifiers by 25 to 50%.
• Reduction of oxygen input and degradation of readily degradable COD during primary treatment by reducing air flow in the grit removal tank, conducting excess sludge directly into the primary clarifiers, and reducing weir height of the primary clarifiers.
• Reduction of oxygen input in anoxic zones by more frequent controls of return sludge pumps.

Digester supernatant treatment: A pilot study is under way to treat digester supernatant with anaerobic ammonium oxidation. In the process, half of the ammonium will be oxidized to nitrite and the remaining ammonium with the nitrite in the anoxic reactor. No organic carbon is required for denitrification; the costs of chemical and energy will be lower than for conventional nitrification and denitrification of the supernatant. If the pilot study is successful, the overall nitrogen removal of the plant will be 75%.

Internal recirculation: Recirculation from the last section of the aeration tanks to the first or second compartments of the anoxic zones is also under study with several objectives in mind:

• To cope with occasionally high concentrations of COD and resulting partially anaerobic conditions in the anoxic zones.
• To supply sufficient nitrate to the anoxic zones when concentrated organic industrial wastewater is to be handled by the plant.
• To enhance phosphorous removal.

Enhanced biological phosphorous removal: If the treatment of digester supernatant can be successfully installed, the first anoxic compartment could be kept in anaerobic conditions which would enhance biological removal of phosphorous.

^* Excerpted from: Siegrist et al., 1999

6.11.3 Case study 3: the reuse of treated effluent in Spain^*

Spain is one of the European countries where water resources are scarce, especially along the Mediterranean cost South of Barcelona and on the Balearan and Canaries islands. Direct and planned reuse of effluent is considered a valid option for augmenting the natural resource though a number of constraints have limited the extent of reuse in the past, among them the degree of treatment of the wastewater and the cost of water conveyance from STPs to the point of use. On the other hand, the following benefits of the reuse of effluent are acknowledged:

• Treated effluent is a valid incremental resource, especially where without reuse, it is discharged into the sea.
• Treated effluent is a valid source for reuse within human settlements where strict criteria will not apply such as those for the quality of drinking water.
• Reduction of water pollution.
• Reduction of energy consumption when reuse is intended in the vicinity of STPs.
• Recovery of nutrient contained in treated effluent.
• Reuse as a major water resource in arid zones, such as on the Balearan and Canaries archipelagoes.

In this context, it is recognized that the amount of treated effluent will increase considerably when the implementation of EU directive 91/271 concerning the treatment of urban wastewater is achieved in 2005 (Section 6.3). At that time, a volume of more than 350 000 m³ will be produced annually of which perhaps one third may be feasibly available for reuse by the year 2012. The major consumer will always be agriculture but other uses are also potentially valid, i.e.:

• Municipal use, e.g. for the irrigation of parks, fire fighting, street cleansing.
• Recreational use, e.g.: irrigation of golf courses, and artificial lakes.
• Recharge of aquifers, combating seawater intrusion and other "ecological" issuses.
• Industrial use, e.g.: flushing and cleaning of materials or use as cooling water.

The need to assure a safe water quality is fully understood and is a major factor in the planning of the reuse of effluent. Strict quality criteria are still needed though it is recognized that in any case, wastewater treatment will involve tertiary technology, especially filtration, microfiltration, physico-chemical processes, disinfection, and/or desalination whenever exposure of people to the treated effluent is possible.

El Cedex, an independent research and development organization under the Ministries of Education, and Environment, has established a data base on the reuse of effluent containing information on the schemes for reuse, the volume of effluent used, application of the effluent, and wastewater treatment provided. There are currently 124 schemes in operation which use a volume of 2 320 000 m³ treated effluent annually. The data base of El Cedex provides detailed information on 41 of the schemes which use 2 080 800 m³ annually or 89% of the total volume reused. Agriculture is the main user with 88.7%. The total volume of effluent is used as follows:

^* Excerpted from: Catalinas & Ortega, 1998

The 41 schemes referred to above vary considerably:

• 25 schemes use effluent solely for agricultural purposes. For all of these, the wastewater is treated by the activated sludge process. It should be noted, however, that tertiary treatment of wastewater is now considered essential prior to the reuse of effluent for the irrigation of certain crops (see below).
• 8 schemes serve agriculture and golfing. Only three of these treat wastewater by activated sludge followed by tertiary treatment by filtration and chlorination or ozonization while the three remaining use activated sludge.
• 4 schemes serve municipal purposes and in a few cases, are combined with agricultural use. All of these apply tertiary treatment in addition to activated sludge.
• Of the remaining 4 schemes, 3 use the effluent for "ecological" purposes and one for cooling. They apply the activated sludge process.

The above information confirms that in the sample of 41 schemes, tertiary treatment of the wastewater is practiced whenever people may have contact with the treated effluent. For the future, however, tertiary treatment is also considered essential as follows although legislation is not yet in place:

• Tertiary treatment plus disinfection for irrigation of ornamental plants in recreational areas with potential contact by people and products eaten raw. In these cases, faecal coliform (MPN) less than 10/100ml and residual chlorine higher than 0.6 and 0.5 mg/l, respectively (after 30 minutes).
• Secondary treatment plus disinfection for the irrigation of cereals, fruit trees etc. with faecal coliform less than 500/100ml and 200/100 ml, and residual chlorine higher than 0.1 and 0.3 mg/l, respectively.
• Tertiary or secondary treatment prior to the reuse for recreational areas with/without contact of the public with the effluent, and faecal coliform less than 200/100ml and 10000/100ml, respectively.
• For cooling, faecal coliform less than 200/100ml and 10000/100ml for, respectively, closed and open systems.

The lack of legislation concerning quality standards for the reuse of effluent is a major problem which constrains the planning of additional schemes for reuse. It has been proposed therefore, the European Union establish a Directive. Other constraining factors include the following:

• The absence of a comprehensive water resources plan with a clear indication of priority areas where the reuse of effluent would be promoted.
• The logistical and financial problems associated with the construction of many STPs with tertiary treatment which would be required.

Nevertheless, 8 new schemes are under construction and for 7 more, tendering is under way. At least 20 more schemes are being planned. Some of those under construction are big schemes with an annual volume of effluent reused of up to 150 000 m³ per year and most are for agricultural irrigation. The scheme under construction for Madrid will reuse 90 000 m³ per year for municipal greenery. Many of the smaller schemes will reuse effluent for parks and golfings.

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