Pipe blocked, nothing moves any more: When waste water systems become clogged.

Waste water system on the verge of clogging

Everyone knows when a waste water system becomes clogged, things can end badly. In the worst case scenario, waste water cannot drain properly and the brown sludge pushes back into the connection points or spills out of floor openings actually intended as drains. The absolute fact of the matter is that municipal waste water treatment plants and other operators of waste water systems are reporting impending clogging with ever greater frequency. What is the reason for this, though, and more importantly, how can this problem be resolved?

Our waste water is not in the best shape as saving water is actually becoming a problem.

Turning off the tap while brushing your teeth or taking a shower instead of a bath, and so on. For years, we were taught (and rightly so) to be as frugal and economical as possible with our precious water. And with success as the last 30 years have seen daily water consumption dropping from almost 150 litres per person to – in some cases – under 120 litres per person per day. This is a 20 percent reduction! While there is no doubt that this is good for the environment, it is difficult for waste water systems because this dramatic cutback in water movement means that our waste water is becoming “thicker”.

Adding to this is the fact that the proportion of solids in the water has been on the rise. Whereas before, mostly toilet paper ended up in waste water, today wet wipes and cleaning cloths, textile fragments, nappies and hygiene articles are also floating about. Even shoes, plastic bottles and metal and plastic chips from industrial production as well as other types of waste are increasingly finding their way into waste water.

It goes without saying that all of this refuse should never be thrown down a toilet or drain, but instead in household or commercial waste bins. Unlike toilet paper, these substances do not decompose easily. Fibres are liable to bunch and get stuck in the pump impellers or waste water treatment plant screens. The consequences can be serious – from significant efficiency losses to clogging leading to unscheduled downtime, extensive cleaning work and potentially high-cost repair work. This is precisely what operators cannot afford: A malfunction not only jeopardises economic efficiency, but also operating reliability. This calls for absolutely reliable waste water pumps and valves that are dependable, energy-efficient and generate low operating costs. And high operating costs of course impact on end consumer prices.

What’s the best recipe against clogging? Always keeping an eye on the overall system.

A waste water plant is often a highly complex system in which each component plays its own special role. This is why no single patent solution exists when it comes to effectively preventing clogging and failures. Rather, the key to successfully operating such a system lies in considering all system-relevant components as all of these areas build on each other and are interdependent. They must therefore always be factored in as a whole if you want to find the optimal solution. These include:

• The waste water to be transported
• The inlet and outlet conditions
• The selection and design of the pump
• The mode of operation of the pump
• Other influencing factors

1. The waste water to be transported

To select the right pump, the first thing to consider is the composition as well as the volume of the waste water to be transported. Is the waste water domestic, commercial or industrial? What is its composition? What is its viscosity? What is the solids, sand and gas content? What volume of water is being moved and how much can the volume fluctuate depending on the time of day and weather? All of these variables have a direct impact on how the system is dimensioned, what impeller type is chosen, the material used and, of course, the operating mode of the pump(s).

2. The inlet and outlet conditions

The main emphasis here is on how the suction and discharge lines are designed – i.e. the dimensioning, built-in components, possible air intake and vortex formation, bends, fasteners, expansion joints, swing check valves, inlet screens, shingle traps, the pump sump design, benching, submergence, switching nodes, etc. All these points can have a direct (negative) impact on the flow behaviour of the waste water. To safeguard the most efficient flow possible, care should be taken to ensure that all lines are designed for optimal flow characteristics. In other words: 

Reductions and extensions should not exceed a defined nominal dimension.
Piping should also be designed to optimise flow (e.g. no tight bends where turbulence can occur). The flow velocity for further transport of solids as well as air pockets should likewise always be considered.

3. The pump selection and design

The core of an efficient waste water system lies in correctly selecting the pump. Waste water pumps and valves from KSB are expertly designed for the fluid handled and the application. High-quality components ensure flawless operation at all times – even under conditions as tough as those found in industrial waste water applications. 

Once the pump type and size have been defined, the next step is to select the pump impeller. KSB offers suitable impeller types for every requirement. The key to making the right selection is the composition of the waste water. What type is involved? Grey water? Black water? Also, what is the gas, sand and dry matter content? From a technical point of view, the size of the free passage and the efficiency are key factors when choosing an impeller. What impeller types are actually available, and what benefits and drawbacks do they have?

Attention should also be paid to determining whether special dischargers such as hospitals or retirement homes are in the catchment area.

Different impeller types and their respective characteristics

The impeller is the actual pumping device for waste water and therefore the “heart” of the pump. Its design therefore makes a major contribution to the operative success of the hydraulic system.

Free-flow or vortex impeller (F impeller)

Pros:

• Very good for handling fluids with a high solids content
• Large free passage
• Also suitable for fibres and rags
• Suitable for high gas contents
• Wear insensitive
• Low vibration excitation

Cons:

• Comparatively poor efficiency
• Accumulation of fibres and textiles increases the power input required

Multi-channel impeller (K impeller)

Pros:

• Very good efficiency
• Low clogging risk thanks to large free passages
• Acceptable wear behaviour 

Cons:

• Hardening of deposits possible upstream and downstream of the impeller

Diagonal single-vane impeller (D impeller)

Pros:

• Good efficiency
• Large free passages
• Suitable for high gas contents
• High occurrence of fibres and rags does not immediately lead to clogging

Cons:

• Complex balancing 
• Wear on vane edge

The newly developed diagonal single-vane impeller D-max

The D-max impeller is a new development that combines the benefits of the conventional D impeller with the efficiency of a K impeller. It achieves an efficiency of 84 % and can be used when a K impeller experiences problems with long fibrous matter.

Besides specially adapted impellers and energy-efficient motors, KSB offers a wide range of installation types as well as a large choice of materials for waste water pumps. So the best solution is available for each waste water application – for optimum efficiency and operating reliability. Learn more about the different impeller types.

4. The mode of operation of the pump

Another very critical factor for an efficient waste water system is the operating mode of the pumps. Above all else, steps must be taken to ensure a sufficiently high flow velocity so that deposits can be avoided. The stand-by pumps in redundant systems should also be started up regularly to avoid prolonged non-operating periods and any uneven distribution of deposits in the pump sump.

Determining the optimum operating range

If the head H (in metres) is chosen in proportional relation to the required waste water flow Q (in m3/h), the optimum operating point is at the intersection of the system characteristic curve and pump characteristic curve. This point must be accurately calculated to define the best operating range in which the pump works at its most efficient level. The operating limits are designated by Qmin and Qmax.

Checklist: The 10 most critical points for waste water systems

1. Always keep an eye on the entire system.
2. Check the waste water composition. Who are the dischargers?
3. What flow velocity should be reached to avoid deposit formation?
4. Upstream of the pump: Check the inlet conditions, in particular the design of the suction line.
5. Choose the correct pump in the correct dimension with a suitable impeller made from an appropriate material.
6. Downstream of the pump: Check the discharge line for optimum flow conditions.
7. Calculate the optimum operating point .
8. Define the most efficient operating mode (Control via frequency inverter? Soft starter? Operating hours? Etc.)
9. Include a power reserve.
10. As required: Mix the contents of the pump sump, ensure backwash into critical areas of the pump sump.