Missing piece in data jigsaw
Envitech installs dedicated event logger with specialised software
The stream in question had no open channel, no exposed closed pipe, no manhole access and no discharge point access. Hence, all normal methods of flow measurement were unsuitable for this application. However, the effluent was pumped from a wet well by means of an alternating duty/standby submersible pump set. This offered the means of measuring flow utilising an Isco pump station monitor, which permits the measurement of flow merely by measuring the time taken to pump down a fixed depth of liquid in a wet well.
The instrument is essentially a dedicated event logger. It will cater for wet
wells using up to three separate pumps and user-specified sequencing. The principle
and practise of operation is very straightforward. The user must determine and
enter the volume between stop/start levels for each pump. In addition the sequence
in which the pumps operate must be entered. A 12V source is required to power
the monitor and a digital input (12 to 120V) for each pump must be provided
when they operate.
The instrument is then able to log each on/off event and the interval between them, as shown in Figure 1.
Flow rates and total flow pumped for each pump cycle are calculated using the equation:
Flow (F m3) = volume between level switches
+ (Inflow rate x pumping time)
Flow rate (m3/hr) = F/pumping time.
Estimates of the inflow rate during pumping are gained in two ways:
a) taking the average of the measured inflow rates immediately before and after
the pump period,
b) performing a trend analysis over a number of complete cycles.
The simpler method (a) is used for the display on the instrument itself whereas the more accurate method (b) is used to process down-loaded raw data using Pumplink software on a PC. In many cases using this method influent flow and pump capacities may be calculated to 1% accuracy.
The food factory application was a comparatively simple one, in that there was one operational pump and one standby pump, operating between a single set of high and low-level switches.
The instrument was installed in a pump control panel adjacent to the wet well and the 4-20 mA flow rate output was directly connected into a local intelligent controller which formed part of the overall building management network.
A company spokesman explained that he was now able to access flow information along with other services management information via the network, enabling reports, trending and analysis to be produced and fed back to the individual department managers. The information would be utilised in an overall site model based on a multistage spreadsheet format, permitting identification and quantification of the different service costs throughout the departments.
He explained that as well as the data production element, the pump station monitor pulse output was being used to pace the Isco water sampler sited by the wet well, thus giving flow proportional sampling. In addition the facility was used, via the building management system, to remotely and automatically inhibit the sampler during periods where factory activity was negligible. Utilisation of the pump station monitor had enabled the last little piece of the wastewater data jigsaw to be slotted into place.
Because the existing data management system on the site has been used, the application does not utilise all the available features built into the instrument. For those locations where such systems are not available, such as small pumping stations and sewage works, it is still possible to log data, produce reports and trends and analyse for pump fault conditions. In fact remote interrogation via modem link is also possible. This latter capability has been used by In Depth Surveys who installed units at approximately eight sewage pumping stations in the vicinity of the Cardiff Barrage site. The objective being to assess the change in ground water infiltration into the sewer as a result of flooding the barrage area. This study is ongoing.
Due to the long-term nature of the study by In Depth Surveys it was decided to utilise panel mount permanent installation procedures. However, for shorter studies the unit may be supplied in a portable carrying case format and the digital inputs may be made via clamp on current sensors, which make temporary installation very simple. As a result it is an ideal tool for short-term flow surveys, routine pump station audits for preventative maintenance purposes and infiltration studies.
Lower flow rates
Recently there has been an increase of interest in the monitoring of comparatively small flow discharges from any source. This stems from the Environment Agency (EA) proposal that by 2005 all effluent discharges of 50m3/day or above should be monitored. This may also be extended to even lower flow rates at a later date. As a result many small sewage works using one or two filter beds with dosing siphons will be covered by the proposals. It is also proposed that the flow measurement should not have an uncertainty of greater than 8% of daily total. For these small total flows it is difficult, especially when there may be long periods during the night of virtually no flow, for conventional open channel flow measurement to meet this criteria. An alternative that presents itself is to utilise the same technology as the pump station monitor but to apply it to the dosing siphon chamber.
Figure 2 shows the water levels just after the siphon has finished operating, i.e. there is no discharge to the filters but there is still an incoming flow. The liquid level rises in the chamber and compresses the air under the dome. This in turn pushes the liquid level down in the siphon pipe and liquid out of the U-tube. After a while the air pressure pushes all the liquid out of the U-tube allowing the liquid in the siphon chamber to rush into the dome and fill the siphon pipe. This will allow sewage to flow onto the filters until the liquid level in the dosing siphon chamber drops below the lip of the dome and permits air to enter again, thus breaking the siphon and repeating the cycle.
This intermittent discharge of sewage from the siphon chamber is completely
analogous to the operation of a pump station wet well, the only difference being
that the motive power for the sewage is provided by hydrostatic head rather
than by a pump.
Two level switches need to be provided at the make and break heights for the siphon and be connected to the pump station monitor via a latching circuit. One or two calibrations of discharge rate need to be made and entered as the nominal pump rate. After that the monitor will recalibrate the discharge rates constantly and apply these to the flow calculation.
Thus one has available the flow rate, totalised flow, and details of the periodicity and siphon operation for the filters being served. A common problem with dosing siphons is that they may not be completely air tight, hence at low flows the sewage may slowly seep up the dome and dribble into the siphon pipe, thus dosing onto the filters with no periodicity and with the likelihood of stationary distribution arms. The ability of trickling filters to process wastewater at conventional loading rates, is completely dependent on intermittent application of the sewage to the localised biofilm. Hence a small continuous flow through stationary distributors will soon saturate the treatment capability of the biofilm immediately below the point of discharge. The inevitable outcome of this is the deterioration of effluent quality, often resulting in breaching of consent limits. Utilisation of the pump station monitor would permit this condition to be diagnosed remotely, allowing maintenance gangs to be dispatched without delay.
Potentially, the pump station monitor will act as a flow recorder and quality
safeguard device at the same time