HOW TO SELECT SEWER LEVEL MONITORING SOLUTIONS

HOW TO SELECT SEWER LEVEL MONITORING SOLUTIONS

 

Sewer level sensors are the “eyes and ears” of water utilities for checking what’s going on in their underground world. Until recently, huge sewer structures were planned and left unattended, waiting for some catastrophic failure. Nowadays, more and more utilities and local Governments closely monitor their costly infrastructure. Some of the benefits from sewer level monitoring are:

▪ Pre-detection of blocking conditions

▪ Sending maintenance teams to the right place at the right time

▪ Preventing environmental catastrophic events such as sewage city spillage

▪ Accounting and costing of wastewater dispensed by industrial plants

▪ Aiding in long-term sewer line planning based on actual usage profiles

This short How-To article is a guideline for utilities planning to implement sewer level monitoring solutions. Four important issues are discussed in this article and listed below:

▪ Durability of the candidate solution

▪ Operation in an explosive atmosphere environment

▪ Communication and data logging

▪ Available measurement data

 

1. Durability

System components which are installed within sewers must withstand severe environmental conditions. Top on the list of hostile threats are:

▪ Physical failure

▪ Penetration of fluids and humidity

Physical failure may originate from sewage solids interfering with measurement mechanism. This type of failure is most apparent with submersed sensors such as mechanical floats and pressure sensors. The use of non-contact sensors such as radar or ultrasonic based sensors reduces the rate of physical failures but does not necessarily eliminate these failures. In particular, when sewer overflows, substances of various sorts may stick to the non-contact sensor and block its radiating face.

 

Penetration of fluids can cause non-reversible failure of the system. Therefore, very high sealing rate is required. Formal sealing rates of IP68 (or NEMA 6P) are not enough in sewer environment. The combination of liquid flow, chemicals and reactive gases are conditions not normally tested in laboratory verifications of IP68 (NEMA 6P) sealing rate.

 

Summary: When checking suitability of sewer monitoring solutions, under the durability category, favor non-contact sensors installed high within the sewer. Also look for additional supporting sealing mechanism such as double sealing or guaranteed scheduled and comprehensive preventive maintenance.

 

2. Explosive gases

Explosive gases may exist in some sewer sites. Methane is a typical gas that may be found in underground sewers. Given proper gas concentrations, a spark may ignite the gas causing it to explode. Sewer monitoring solution should have an option for safe use in explosive atmospheres. The additional requirements for an explosive safe system will increase price and may also reduce performance. Therefore, standard systems are preferred in sewer sites where there is no threat of explosive gasses. Systems that may be used safely in explosive atmosphere must be accompanied by a proper certification from one or more of the international or local standardization bodies. The most recognized certifications are:

▪ ATEX (European standard)

▪ FM (American standard)

▪ IECEx (International)

However, other local regulations may apply such as:

▪ NEPSI (China)

▪ GOST (Russia)

Methods of protection vary: from explosive-proof enclosures to Intrinsic Safe electronic circuits. It is important to note that certification must hold for the system as a whole. Deploying system components, where each component is certified individually, does not guarantee that the complete solution is certified.

 

3. Communications and data logging

Information is usually extracted from the sewer sensor in one of the following manners:

▪ Local logging accompanied by periodic visits to the sewer for data download

▪ Wireless transmission of the RF Radio type

▪ Wireless transmissions of the Cellular type

Local logging is a reliable technique as it does not involve wireless communications. Local loggers add to the system costs but usually not as much as wireless transmitters and communication infrastructure. The major downside is the extra handling and human involvement in downloading logging data. This involvement increases costs and is prone to errors.

 

Wireless transmissions are classified according to their communication infrastructure. The use of RF radio infrastructure involves dedicated communication infrastructure and the use of regulated RF frequencies. The allowed frequencies for this application may differ at different countries. In some cases, RF communication infrastructure is a two-tier structure where the fielded sensor transmits to a local hub and the hub transmits (by wires or wireless) to a central location. The main advantage of RF Radio based solutions are reliability and tailored coverage. The main disadvantage is the need to setup a dedicated infrastructure and local regulation challenges.

 

Alternative to RF Radio transmissions is the use of an existing cellular infrastructure. The big upside in this scheme originates from taking advantage of an existing communication infrastructure hence lower costs. Sewers are normally located in urban areas where cellular coverage is good. The downside for the use of cellular communications is apparent in deep or metallically enclosed sewers. In these sites, or sites where the enclosing structures is combined concrete/metallic structures, underground cellular coverage may be low or non-existent. External antennas may, in some cases, circumvent these problems.

 

4. Measurement data

Sewer level sensors are not necessarily limited to level measurements. Additional data may expand the capabilities of the sensor:

▪ Level

▪ Flow

▪ Temperature

▪ Other generic-type sensors

Sewer level information is vital for identifying overflow conditions, blockages, overuse of pipes, long term planning and more.

 

Sewer flow data (for example in terms of cubic meter per hour) and sewer volume data (in terms of cubic meter) is an additional information building block. Flow information may be extracted from level information in one of two methods:

 

▪ Use of flumes or weirs inside the sewer pipes or under manholes

▪ Use of Manning formula

Temperature sensing also expands the usefulness of the system. Short term increase in temperature can indicate, for example, industrial waste being dumped into the sewer.

 

Finally, the ability to connect to additional sensors through some standard protocol may further increase the effectiveness of the system. These other sensor may be of the chemical type (such as PH) or biological type. However, it should be noted that independent sensors, ready to connect to a general purpose terminal, are rarely available on the market.