Anything but 'run of the mill'
Effluent monitoring can contribute to a company's environmental success, as paper manufacturer, Aylesford Newsprint discovered with the Lange UVAS probePaper accounts for around 20 per cent of UK waste, so the importance of pulp and paper manufacturers demonstrating environmental commitment cannot be over-emphasised. In the production of its 400,000 tonnes per annum of recycled premium grade paper, Kent manufacturer, Aylesford Newsprint's water treatment plant deals with approximately 16,500m3 per day of process water mainly from the fibre preparation plant. Effluent from the paper mill is first filtered (primary treatment) and then cooled before entering the activated sludge plant before clarification and discharge to the River Medway.
An aqueous solution of cationic polyacrylamide is injected prior to three parallel rotating drum filters to agglomerate the suspended solids and assist drainage. The number of filters in service depends on the current demand. The separated suspended solids are discharged via a chute into the mixed sludge sump. The filtered water is deposited in the primary treated water (PTE) sump. The temperature of the PTE can be as high as 45oC when the mill operation is stable. This temperature could inhibit secondary activity and cause the consent limit on final water temperature to be threatened. Therefore the PTE is cooled to about 27oC in a 'single pass open evaporative cooling tower' consisting of two cells with variable speed fans. Each cell is designed to remove about 12.5MW of thermal energy and one cell is usually sufficient to achieve the cooling duty. Secondary treatment
The plant consists of up to seven aeration tanks. Oxygen is obtained from either a liquid storage vessel or an on site oxygen generator. Two clarifiers are used for separation of the activated sludge from the cleaned effluent, which is then discharged to the River Medway. The retention time in the aeration section of the plant is about 10 hours (depending on hydraulic throughput). The selectors can be run anoxic or aerobic, which means that there are areas of 'fast uptake' (or adherence to the sludge). Under normal operating conditions the aerobic plant removes approximately 85 per cent of the incoming COD and 98 per cent of the BOD, giving BOD figures of less than 10mg/l and CODs of about 250mg/l. These values are considered normal for a de-inking mill producing newsprint. The suspended solid level is generally about 15mg/l.Tertiary treatment
There are no water recycling tertiary treatment stages but there is a drum filter available to treat solids carryover from the clarifiers if required. This is normally bypassed. Solids separated in this process are transferred into the mixed sludge sump. The hard COD remaining in the final effluent is typically 90 per cent lignin with the remainder being extractives, consisting of chemicals such as waxes, resins and oils, normally from adhesives attached to the recovered paper. There are a number of factors affecting the choice of instrumentation for monitoring of discharges.
Monitoring may be necessary for operational reasons, or it may be necessary for compliance purposes. Operational monitoring should demonstrate that genuinely valid measurements of water quality are being recorded in a manner that is consistent with accepted procedures. However, measurements to demonstrate compliance with a specific discharge consent should take place in the units and format of the consent. For example, BOD is a common measurement of pollution load in discharges to controlled waters, and COD is a common measurement for discharges to sewer. This situation has not changed since the introduction of the IPPC Directive.
Frank Holton of Aylesford Newsprint explains how Lange's UVAS probe monitors the organic loading in the primary treated effluent and in the final discharge to river: "These probes provide 'operational monitoring' ensuring that environmental performance is monitored continuously, so that any potential problems are quickly identified", he says.
For the past three years Aylesford has deployed Lange UV probes to measure organics in primary and final effluent streams. However, the company had previously used traditional TOC monitors that required a high level of maintenance and often failed. Lange UK was therefore invited to recommend a more reliable alternative. The UVAS monitors offered an effective solution - they are robust, self-cleaning and require no calibration.
The UVAS measures UV (254nm) extinction (i.e. the proportion of light that is absorbed by dissolved organics in the effluent) in-situ, by flashing two beams of light across an aperture in the probe housing. The effect of turbidity is compensated by a reference measurement. The resulting UVAS reading is known as the SAC (spectral absorption coefficient).
Numerous studies have proved the correlation between SAC and COD or TOC. Baumann and Krauth obtained correlation factors of 0.94 and 0.98 for COD and TOC measurements respectively, in the primary settling tank. Matsché and Stumwöhrer also show that "UV-absorption has an excellent correlation to COD in wastewater treatment plants".
UVAS probes can be connected to a multifunctional display unit, the Multi Unit Plus, from which the continuously measured and logged value is immediately available. Users are able to request hourly, daily, weekly, or monthly progress reports, which can be shown graphically or downloaded by RS232. Other Lange monitors, such as pH, turbidity, dissolved oxygen and sludge can be included to form a complete monitoring system.
SAC measurements are routine at many different types of treatment plant, and are deployed at several points in the treatment process, providing valuable control information for example in intake, activated sludge, and discharge processes. Aylesford Newsprint believes that: "From an operator's point of view the UVAS system is simple to operate, reliable, robust and requires very little maintenance, and consequently fulfils a vital role in the protection of the River Medway".