Not all villas, labour camps and remote site offices are quite located in the middle of a municipal sewer line in the UAE. Once you’re out of the main part of either Dubai or Abu Dhabi, you’ll find there to be ample examples of properties that are quietly collecting their wastewater in a buried concrete tank. That tank is actually doing quite unspectacular tasks: collecting, separating and safely storing sewage until it can be treated or removed – and a wrong design does not only lead to a bad smell. Contaminates the soil, leads to structural failure, and can lead to municipal fines which makes the initial investment for the tank appear low.
Let’s take a look at the uses of precast concrete sewerage collection tanks, why concrete is the material of choice in this climate and what makes a properly specified system a system that doesn’t create a problem.
The Advantages of Precast Concrete Sewerage Collection Tank Still Prevail in the UAE Soil Conditions
That’s not to say that there are no plastic and fiberglass tanks, but their strength and impermeability make them well suited to withstanding the weight of wastewater and the corrosive salt content of the UAE’s soil.
Dirt, extreme heat levels in summer and salty ground water intrusion can make cheaper alternatives last much less than what is expected by the homeowner in the UAE, where the climate is often quite dry, compared to what they might expect in a milder climate.
A concrete structure is inherently water tight, unlike plastic and fiberglass which need to be specially processed to be water tight, so that is a definite plus for a wastewater structure whose sole duty is to contain sewage until it can be safely processed.
What Sewerage Collection Tank Does?
The principle of operation is simple: the tank separates solids and liquids, allowing some treatment and safe disposal. Simple mechanism but really critical for hygiene and public health, waste water enters the tank, solids settle at the bottom and treated water is finally discharged.
There is a common misconception that in a developed country such as the UAE, there is no need for any decentralized sewage system. Collection tanks fill those gaps of infrastructure, remote areas may not be serviced by sewers, temporary projects require quick solutions, and small-scale developments warrant cost-effective options.
1. Single Chamber vs Double Chamber Design
The first actual design decision is to treat wastewater in a single chamber concrete septic tank, allowing suspended solids to settle at the bottom as sludge, fats and oils to float on top as scum and the partially treated effluent to pass out the T-shaped outlet which prevents the scum and sludge from escaping.
The economics of using single chamber versus double chamber systems are simple, but the convenience is long-term. Single chamber systems have lower initial costs and are easier to install with fewer parts, but double chamber systems have fewer pumping, drainfield replacement, and maintenance costs over time.
The size of each will depend on how you’re using it: a single chamber tank will usually be sufficient for a small two bedroom villa, a temporary site office or a holiday home, whilst the increased treatment capacity of a double chamber tank is likely to justify its use in larger residential or commercial sites.
2. Baffles and Internal Flow Control
The inside dimensions of the tank are as important as the capacity. At the inlet, there are fittings which are tee-shaped or walls that stop incoming wastewater from disturbing settled solids layer and washing solids directly to the outlet, those fittings or walls are called baffles.
The placement of baffles or tees at the influent pipe is another small detail that can have a tremendous impact on the ability of a tank to remove solids from liquid during its use life.
Industry best practice requires baffles or tees to be placed at the influent pipe. If you omit this detail, you’ll eventually get a tank that may look fine, but will start out badly.
3. Structural Loading and Design Standards
Underground tanks do not only store wastewater, they also transport loads from the soil and from all loads or forces acting on or above the tank. These loads are calculated using the ASTM C890, Standard Practice for Minimum Structural Design Loading for Monolithic or Sectional Precast Concrete Water and Wastewater Structures.
A supplier that can make this claim by name—specifying ASTM C1227, Standard Specification for Precast Concrete Septic Tanks, and ASTM C913, Standard Specification for Precast Concrete Water and Wastewater Structures—is a supplier to be taken seriously.
4. Installation, Bedding and Ground Preparation
Even a well-engineered tank can have a short life span if the base under the tank has not been prepared correctly, and this is often rushed. The base beneath the tank should be flat and excavated to the correct size and depth and topped with 5-6 inches of pea gravel or sand over a well-compacted, levelled base.
The UAE experiences shifting sand after rain or during extreme temperature changes, and proper compaction and level is not a convenience; it’s what actually keeps a heavy precast tank in place for a number of years.
5. Inlet Pipe Sizing and Slope
The pipe that brings the effluent from the property into the tank needs to be 15 cm in diameter and have a slope of at least 2 per cent (i.e. 2 cm fall for every 100 cm of pipe run).
Even a perfectly constructed tank with the right water level will fail to function correctly if the pipe work that supplies it is incorrect.
There’s no random number for that slope. If too flat, the solids will not travel to the tank and block the pipe where they are meant to go, rather than on their way to the tank. A detail that can easily be missed on a drawing, but is expensive once the pipe is buried and backfilled.
6. Waterproofing and Corrosion Protection
The sealing of joints between precast sections is as important as the concrete in the mix and a seamless and watertight structure is one that helps prevent soil and groundwater contamination.
Since the soils in the UAE are predominantly saline, the corrosion protection of all the joints and reinforcement details is not a choice; it’s a necessity to ensure the tank’s longevity of decades, rather than years.
The Real Challenge Isn’t the Tank, It’s Matching the System to the Site
These are the things that most sales conversations overlook. The selection of the single or double chamber, sizing the tank appropriately and establishing the standards correctly are all engineering problems that can be solved.
The more difficult step is to match the entire system with what is truly needed at the site, rather than the cheapest or most familiar one.
It’s not necessary to use a double chamber system in a small holiday home and a labour camp with dozens of workers can’t play catch up with an undersized single chamber tank for a two bedroom villa.
If you miss this match, it won’t show up right away. It manifests itself months later with frequent pumping calls, slow drainage or in the most severe cases, with an over-flowing tank, where the tank is not supposed to get in contact with ground.
The Bottom Line on Precast Concrete Sewerage Collection Tank
The properties located outside the municipal sewerage connection area in the UAE are exposed to high temperatures, salty soil and structural loads—factors that make it a better option to choose precast concrete sewerage collection tanks over lighter alternatives.
It is the details that make all the difference, chamber configuration, baffle placement, proper bedding, correct inlet slope, and standards such as ASTM C1227 and ASTM C890, that make all the difference between a tank that operates quietly for years, or a tank that becomes a recurring maintenance issue within a couple years.
