There are more than 2 million sewage Lift or Pump stations in the US. All work on the same principle and with the same objective of moving sewage from one level to a higher elevation. Their installation costs range from $150,000 (20 gallons per minute) to $1.5M (100,000 gallons per minute) generally based on capacity and complexity. Of course the pump technology has come a long way in recent years, but the purpose of this article is to focus on a small component which has also received significant development in the past few years and is essential to the pump control and reliability of the station. This is the level sensor. A typical schematic for a sewage lift station featuring the level sensor (pressure transmitter / level transmitter) and its associated hardware is shown in the figure 1 below.
The level sensor
The purpose of the level sensor is to provide an electrical feedback to the pump as to when to switch on and off. Traditionally, floats have been used which simply provide an on and off signal to the pump at the high and low levels. Bubbler systems have also been utilized although they create increased maintenance challenges with the requirement of a continual gas flow. Today there are many sensor technologies for measuring liquid level such as Radar, ultrasonic and conductive. However, these are either too high in price for a relatively simple lift station or unreliable due to the operating environment. In recent years, submerged hydrostatic pressure transmitters have been developed to withstand the environmental conditions and provide continuous monitoring for enhancement of the control with increased long term reliability.
A number of manufacturers such as STS have developed dedicated sensors for this application. An example of this is the ATM/K/N as shown below. There are many features which have been specifically designed into this level/pressure transmitter to overcome the challenges faced in sewage lift stations.
Image 1: ATM/K/N - Submersible level sensor with ceramic measuring cell
As many lift stations are located in very inaccessible places the overwhelming requirement is for reliability. This requires a clean design with high integrity seals. Due to the nature of the effluent, the sensing element must be exposed to avoid clogging.
This problem is also becoming more important due to the increase in FOG (fats, oil and grease) associated with fast food restaurants. The use of ceramic capacitive sensing technology provides a rugged open face sensor while having the ability to achieve high accuracy, better than 0.1%, down to sewage levels of just a few inches of water. The technology also provides a very high overpressure of at least 3x the rated range without any degradation of the sensor performance. This protects the transmitter against damage due to overflow or back pressures. The laser welded 1" diameter housing is generally made from 316L stainless steel, although titanium is often preferred where the effluent is more corrosive.
A further design feature is the electrical connection.Various electrical outputs are required including the most popular 4-20 mA 2-wire loop power or 1-5 Volts. In some cases users wish to adjust the level transmitter and this can be achieved via digital communication featuring a full scale range turndown to 10% of the originally specified range. These transmitters can be provided with a full scale preset range to suit any sewage lift station. In lift stations where hazardous gases exist, transmitters can be certified FM intrinsically safe for use in Class I, II & III, Div. I, Groups A, B, C, D, E, F & G.
The cable termination is also important, not only to provide connection to the control system and pump, but also to provide an outlet for the breather tube to the atmospheric pressure. This is vital to ensure the correct operation of the transmitter which would otherwise be affected by changes in barometric pressures. However, this reference breather tube must be protected from ingress of moisture. There are many techniques for this, such as the use of desiccant within the termination enclosure, to enhance the long term reliability of the transmitter. STS has developed a sealed Mylar enclosure which requires zero maintenance and does not rely on the use of desiccants or consumables.
Because the submersible level transmitter is relatively light in weight and it is preferred to position the transmitter a few inches from the tank bottom, it is fairly common to use sink weights. This type of sink weight is sometimes called a "bird cage" and, in the case of the STS transmitter, can be removed from the transmitter if necessary. In other cases, the "bird cage" is integral to the transmitter.
The high integrity well developed submersible pressure transmitters of today provide very reliable, zero maintenance, level monitoring and pump control for sewage lift stations and deep well monitoring. These hydrostatic level measuring transmitters are continually monitoring the sewage level, and with the enhancements in the associated control systems, provide information related to pump performance and general health monitoring of the facility.