The Need for Rat Control in Sewers

The need for rat control in sewers is substantial, and it is worth examining why this is the case. Primarily, rats living in sewers do not always remain there. As in many habitats, there is a propensity for animals to wander, especially when the population grows too large for the resources of food or lodgings. 

Furthermore, migrations may be caused by inundations after heavy rainfall, or from other forms of disruption. This could result in the emergence of a new rat colony in an area that had only recently been cleared with much effort. In fact, many of the fresh rat infestations noticed by local authorities are connected to faulty sewage systems. It must be remembered that rat activity in drains does not always relate to activity in sewers, nonetheless it is wise to establish the degree of correlation between them.

Rat Control in Sewers
Rat Control in Sewers

Impacts of Rat Migration on Human Health

From a public health perspective, a far more concerning kind of rat migration between the sewer and the exterior is when rats inhabit the sewers and make daily trips to the surface for sustenance. In certain instances, rats have even been able to outwit correctly functioning, however poorly configured, water seals in bathroom toilets in residential dwellings.

Damage Caused by Rats to Structures 

Secondly, rats living in sewers can damage structures since they don’t reside in the sewers. Usually, they create holes in the ground near the sewers, going in through faulty joints or deteriorated brickwork. The excavated earth can eventually block the hole, leading to a subsidence in the road above and even breaking gas and water mains. Furthermore, cross contamination of nearby telephone and cable ducts has been observed, resulting in extreme damage to wiring and service disruptions for telephones and cables.

Control of Rat Population in Sewers

Lastly, rodents play a critical role as carriers of different illnesses and rats that come from sewers may spread disease to humans. They can be vectors of a variety of microorganisms from the sewer system, thus making it possible for them to cause harm. It is beneficial to attempt to reduce the rat population of a specific sewer in order to reduce the potential damage. Although removing them completely may not be possible, taking steps to control their numbers is still worth it.

The Environment

For rats to thrive in sewers, they require two essential elements: food and shelter. Even if one of them is abundant, it is pointless if the other is lacking. Water is likely to be an inexhaustible resource! Rats can resist a few days of starvation with no major damage, but if they are to reproduce in any substantial numbers, they need a regular supply of food. Additionally, each rat needs its own refuge, particularly females with their litters, as overcrowding can lead to the death of the young, either due to trampling or harbouring predators.

Assessing the Local Sewer System

The diversity of sewer systems across various towns requires a variety of approaches for rodent control. Every system is unique and demands a separate assessment of its structure, design and layout in order to identify potential points of entry and sources of food and shelter for rats. In some areas, where the soil and storm-water systems are separate, rats are unlikely to be found in the storm-water sewers. In other locations, however, combined or partially combined systems may be used and rats can inhabit both. To effectively control rat populations, it is important to understand the local sewer system and identify areas where they can get food and shelter.

Structural Components of Sewers 

Small sewers, ranging from 15 cm to 60 cm in diameter, are typically composed of glazed stone-ware, fireclay pipes, or pipes made from artificial substances, and are laid end-to-end. Those with a diameter of about 90 cm or greater are usually of concrete, brick, brick-and-concrete, or cast-iron, and are usually circular in cross-section. Other shapes, such as rectangular, trapezoid, and ovoid are also seen, particularly in old “brick-barrel” sewers.

Rats have been known to traverse the sides of larger sewers by leaping back and forth over the current, even when it is too strong for them to swim. However, they are only plentiful in areas where swimming is not needed during certain times of day.

Sewer Design and Condition

Rats can often navigate brick-barrel sewers with their hind legs on one tier and the forelegs on the higher tier, above the floor level. This movement, which is similar to a crab-like scurry, is enabled by the outward jutting of each row of bricks at the sides. Newer systems are likely to be less prone to fault and breakage, thus providing a less inviting habitat for rats.

Today’s drains and sewers are constructed using advanced techniques, often utilising man-made materials, which create a smoother surface than in the past. This makes it harder for rats to find a comfortable home, as the environment is less hospitable and less varied.

The design and condition of the sewer system can have a major impact on the number of rats that can find harborage. In many places, drains from buildings and land on the surface link to the sewer via an “interceptor trap” with a water-seal, though some areas may lack these features. Without the seal, this could provide more opportunities for rats to gain refuge.


The presence of a water-seal in a drain does not necessarily mean that it is functioning correctly. If the drain is only used infrequently, the water-seal may evaporate. In drains that are connected to larger sewers, hinged flaps are used to seal them. These flaps will only open when sewage is discharged. If the flap is not working correctly, it must be repaired or replaced to prevent rodents from entering the drain.


Navigating Sewers

Rats can take advantage of bad jointing between drains and sewers, or of a disused drain that has been left behind due to demolition of a property. These scenarios create a suitable habitat for the rodents if they can dig their way out into the subsoil. Additionally, dead-ends and unused sewers in areas with extensive redevelopment also make for a welcoming home for rats.


The sewers at the upstream end typically have branches that all slope down to the out-fall at the lowest point in the area. However, this is not always feasible. In some cases, sewage may collect at some point before the out-fall and require pumps to push the effluent to its destination. This is especially challenging when the volume of sewage varies from hour to hour, since the pumps may be unable to handle the flow at times, leading to backing-up. This can also cause the sewer to be temporarily impassable to rats. When the flow returns to normal, there may be a lot of edible debris left behind. In large cities, additional intercepting channels may be constructed that terminate miles outside the city, tapping into the original sewers.


The flow of water at the highest points of a sewer system is typically sporadic, which can lead to food particles being left behind and not enough to support a large rat population. As more and more branches of the sewer link up, the flow is usually stronger and more consistent. However, it is still subject to notable changes in level depending on the surface, amount of silt, and overall strength of the flow. As the sewer network develops and the cross-section of the pipes expands, the flow of water can become so deep and strong that rats cannot survive in the area.

Manholes and Benching

At various points along sewers, “manholes” provide access for inspection. Generally, a manhole comprises a vertical shaft, either square or circular in shape, and wide enough for a person to climb in. In some areas with a high traffic density, the manhole located directly above the sewer may be replaced by a side-entrance on the pavement. This features a similar shaft with a passage at the bottom linking to the sewer. At the bottom of either the manhole or side-entrance, the upper part of the sewer is removed and the remaining part, the invert, typically has a gently sloping “benching” on either side of it above the normal water level. Rodents often use this area as a resting spot, making it a useful place for poison-baiting. Unfortunately, not all manhole chambers are equipped with “benching”.


Advantages of Sewers to Rat Populations


The gap between manhole chambers may differ significantly. In newer sewer systems, the recommended maximum range is about 100m, but older systems may have a separation of over half-a-mile. In general, sewers provide a desirable environment for rats due to the temperature remaining relatively constant throughout the year. Furthermore, there are no predators in sewers and the abundance of food and hiding spots can facilitate the growth of rat populations.


Sewer Rat Populations


Rats are highly apt to flourish near restaurants, markets, canteens, grocery stores, and apartment buildings that don’t have proper garbage disposal systems, in sewers that have gone through a lot of construction and the ancient drains have not been secured correctly or are insufficiently self-cleansing and when there is a lack of upkeep; and where the sewage is not sealed off.


Rats are unlikely to become plentiful in sewers that are properly maintained, of small size, and serve areas with a low population density. Additionally, sewers in some coastal cities, which are exposed to flooding after moderate rainfalls or inundated with tidal waters twice a day, also tend to have a lower rat population.


Rat Migration in Sewers

No matter what the circumstances of a given system may be, the number of rats that can inhabit it is restricted by the competition for sustenance and shelter; even so, this limit may change with time. The specifics of this process are still not fully understood, however it is evident that in many cases the population is kept in check through migration – to neighbouring ecosystems or to the open air.


It is possible that prior to migration, the surge in population can create frequent clashes between individuals, due to the linear nature of the environment making it hard to avert. Such heightened conflicts could easily have an influence on reproductive achievement – as has been demonstrated in other comparable circumstances on the ground. It could also lead to fatalities from battling.


It is of great relevance to gain knowledge about the speed of recovery when rat populations have been reduced. Fortunately, there is some evidence available. It appears that a reasonable assessment of the natural rate of expansion of sprouting rat populations in sewers may be around 3% per week. Yet, higher rates (up to 11-12%) have been registered in certain cases, and this could be attributed to the influx of rats from adjoining sewers and from other places. 

Behaviour of Rats in Sewers

The behaviour of individual rats in sewers has been studied in some detail. It appears that each colony typically inhabits a single area and its members go on foraging trips of a limited distance in search of food. This is evident when people walking through sewers observe the rats’ reactions, when they find rat droppings that have been dyed from eating baited food, and from experiments involving the placing of bait in large, open sewers. The distance that individual rats will travel can vary, but it is thought that they may roam farther when food is scarce and may even move more in certain days.

Home Range of Sewer Rats

A study involving a colony of rats showed that they were able to travel up to 200m in one direction and 140m in another from a baited manhole. In a similar brick-barrel sewer, it was observed that the rats were able to stay within a 140m range between two manholes and rarely visited the latter. This home range is likely to be seen in sewers that are conducive to rats, or those that have regular traffic between the sewer and surface. This poses a challenge when poison baiting is conducted only at manholes, as well as in other sewers where rats travel greater distances and the manholes are far apart.


Food Preferences of Sewer Rats

Evidence suggests that sewer rats are often less picky than their counterparts above ground when it comes to food. Reports have indicated that rats may not be inclined to eat mouldy bait if they are not in desperate need of food. However, it is possible that sewer rats may be more likely to quickly overcome their “new object reactions” when faced with palatable baits. This could suggest that direct poisoning could be more successful in sewers than above ground. Nevertheless, this does not mean that baits and poisons can be removed more quickly, as the maximum consumption of bait may take up to nine days in some cases.


Challenges of Poison Baiting in Sewers

It has been observed that in a single sewer, even after 18 days of extra baiting, rats still got more than 20% of their diet from other sources. This could be due to the fact that they may have been deterred from feeding at the baiting points due to conflict between the members of the same colony, or because more than one colony was present near each manhole. Additionally, rats in sewers may not all feed at the same time, adding to the need for additional baiting, especially when using the more toxic second-generation anticoagulants, brodifacoum and flocoumafen.


Control Methods Available

The use of poison baits is the only effective method to eradicate rats in sewers. The baits should be placed in manholes, but they can also be set along the length of sewers that are wide enough to walk through.

Access and Rescue Considerations 

It is not recommended to ever enter a sewer without written permission and proper training. Sewers present a number of hazardous risks such as dangerous gases, making access and rescue operations very difficult. Unfortunately, a number of people have died due to unauthorised entry in the past.

Unsuccessful Control Techniques 

Various strategies for dealing with rats in manholes have been suggested, such as gassing, flooding, the use of poisoned “sticky-boards,” and the application of poisoned foam to the benches. Unfortunately, none of these techniques have been effective, and the potential for lethal gases to escape the sewers and infiltrate buildings has stopped the use of highly toxic gases in this context.


Professional Rat Control Services

It is recommended to hire a professional rat control service company. They can provide a range of services, from identifying the source of the problem to providing traps, poisons, and extraction services. Professional services should always be hired for jobs that require access to the sewers, as they are trained to deal with the hazardous conditions

At Bon Accord, we offer comprehensive pest control solutions for individuals and businesses alike. Our team of qualified pest control technicians have the expertise needed to identify and quickly eliminate even the toughest pest infestations. We guarantee that your home or business will be free of pests when we are done. Rat control and extermination services you can rely on.

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