Samplers on the move
Ian May looks at how Yorkshire Water evaluated a portable samplerProcuring a sampler is one thing - but for today's water companies, a portable sampler must meet the stringent requirements of E32 - the Automatic Sampling Equipment specification for the Urban Wastewater Treatment Directive (UWWTD). The following article is edited from a comprehensive report by Ian May at Yorkshire Water on how the firm went about evaluating an E32 compliant portable sampler.
Under the UWWTD, a number of small WwTWs will have to begin effluent (and in some cases, influent) testing, in order to demonstrate compliance with discharge consents. Sampling regimes may be based either on absolute levels of a given analyte in the effluent or on percentage removal across the plant.
One option to facilitate this would be to install fixed sampling units at every site. However, this would require a significant financial investment, with one (for absolute level sampling) or two (for percent removal sampling) needed at each site. Since the legislation requires a varying number of sampling periods based on the flow treated by a given site, some very small sites may only require to use a fixed sampler every three months.
The sampling events required are detailed in Table 1. The alternative to using fixed samplers is to select portable units, which are more cost-effective and able to serve the sampling requirements of a large number of sites in turn. The potential savings on capital costs for Yorkshire Water are very significant - between £400,000 and £500,000. However, portable samplers must comply with the Environment Agency's (EA's) E32 compliance specification (EA, 2001), which includes provisions for sample temperature control. Briefly, wastewater samples for demonstration of UWWTD compliance must be stored, both during and after sampling, at a mean temperature of 0-5°C, with no freezing of the sample. The temperature range must be maintained for a minimum of 24 hours after sampling comes to an end. Samplers must be able to hold this temperature range in ambient temperatures between -10 to 40°C. Fixed samplers achieve this by including a small refrigerator or heater to control the temperature both during sampling and until sample collection, with no ice formation. In order to establish the compliance of its P2 Coolbox sampler with the E32 specifications, we noted that Aquamatic commissioned WRc to carry out work in this respect. The complete requirements of E32 compliance are extensive, covering areas such as sample line velocity, power supply reliability and lift height. The sampler was found to provide good control over sample temperature, with the samples generally being held in the 1-3°C range.
In order for comparison to be made between fixed and portable samplers, a brief description of some key aspects of the use of both types of samplers is included in Table 2. The E32 specification is particularly important for biologically active samples (including most sewage treatment effluents) and where consent parameters include biological oxygen demand (BOD), which is liable to large and rapid changes at temperatures above 5°C. Operational requirements are included to ensure samplers are designed and built to a reasonable standard for their intended use. For example:
- the chemical composition of the sampled medium must not be significantly altered by the sampling process,
- since the specifications are vague with regard to the integrity of the chemical composition of samples, the WRc chose to evaluate BOD, COD, suspended solids, total nitrogen, and total phosphorus.
Environmental requirements are concerned with sample temperature control. For samplers that include a means of controlling the sample temperature, the specifications state that: "When operated in an ambient temperature rated range of use of -10°C to +40°C and a process fluid temperature rated use of +1°C to +25°C - the mean temperature of the sample shall be maintained within the range 0°C to +5°C during the sampling period. After completion of the sampling period, the sample temperature shall remain within the range 0-5°C for a minimum period of 24 hours. The design of the sample temperature control shall ensure ice does not form in the sample." The Aquacell P2 sampler is based on a modular design that allows a single design of sampler unit to be used with a number of different base units - each designed for a different application. The Coolbox base has been designed specifically for UWWTD applications.
The system is designed to prevent samples from freezing by using the latent heat of the ice - if the ambient temperature falls below 0°C, the small amount of heat energy stored in the ice is released, keeping the sample within the 0-5°C range. Bottlers are unavailable for this base unit, so only composite or single grab samples may be taken. The sampler has been designed to require as little maintenance as possible. Occasionally, it may be necessary to clean the sample chamber electrodes or change the desiccant bag.
Both tasks are very simple and take around five minutes each to complete. During the winter sampling regime, the sampler was run for 20 consecutive 24-hour composite sampling periods at a Yorkshire Water WwTW near Sheffield. The plant treats around 175Ml/d of domestic and trade sewage using two activated sludge plants.
Around half of the samples were taken from the influent sewage, after fine screening and grit removal but prior to primary settlement. The remainder were taken from the final effluent sampling point. For comparison, the fixed on-site samplers (made by a different sampler manufacturer), installed at the sampling points, were run simultaneously with the Aquamatic unit. The samples from each were analysed for four common wastewater parameters - five-day biological oxygen demand, chemical oxygen demand, suspended solids and ammoniacal nitrogen.
The results were then compared. Samples were taken for analysis by refrigerated courier. Temperature and event logs for the Aquamatic sampler were preserved. During the summer sampling regime, the process was repeated. However, several samples were, for convenience, collected from Northallerton WwTW. This two-stream works treats around 8Ml/d of domestic and trade wastewater using two activated sludge plants and biological filters.
The effluent sampling machine at Northallerton is made by a different manufacturer, while the influent sampler is an Aquamatic fixed sampler. These results cover one winter sampling period (November 2001 to January 2002) and one summer sampling data period (August-September 2002). During the trial, all temperature logs were recorded. The sampler never recorded a temperature outside the 0-5°C range. The fixed units did not suffer any breakdowns during the trial, nor did the sample temperature fall outside the 0-5°C range. The mean sample temperature was 0.1°C.
The sampler was loaned for a period to the north area sampling team, who would be the end users of the Aquamatic should it be selected for general use. During this period, it was used as a reserve back-up sampler when fixed units broke down or sufficient portable samplers were unavailable. In conclusion:
- the sampler has been found to perform identically during winter and summer conditions. This is in keeping with the findings of the WRc investigation, which found the sampler was able to achieve a sample temperature of 0-4°C over the range of temperatures expected from the British climate,
- during the course of this trial, the sampler has performed reliably. No breakdowns have occurred and the only maintenance required thus far has been the replacement of a desiccant bag,
- the sampler has reliably carried out a number of 24-hour sampling runs with no problems. The sampler has also demonstrated the ability to hold the E32 specified temperature range for a further 24 hours dwell time after sampling,
- the interface is user friendly and the sampler has proved easy to use and programme. It was found to take less than one hour to become proficient at programming the sampler,
- the sampler has successfully sampled screened crude wastewater without trouble - a filter built onto the end of the intake hose has prevented ragging from causing any problems. The sampler has also successfully taken composite samples of final effluent from a wet well approximately 6m deep without any problems,
- this is a ruggedly built sampler that is straightforward to use. It has been shown by WRc to comply with the EA's E32 specification in all respects and has further been demonstrated in this report to meet the requirements of this with regard to sample temperature control and integrity,
- based on the results of this study, the Aquamatic P2 Coolbox sampler is recommended for use in UWWTD compliance sampling. By using the sample temperature logging facility, samples will be auditable by the EA for temperature compliance