Maintaining filters and backwash pumps in a water treatment plant

In a conventional water treatment plant, sand filters are the last barrier before disinfection. If coagulation and settling have done their job, the filter catches the remaining particles and filtered water turbidity stays below one NTU. But a filter only works well if it is washed on time and properly, and washing depends on a set of pumps, blowers and valves that usually get less attention than the main equipment.

This note is a short review of maintaining that set, from the point of view of someone who has to plan and record the work.

The odd duty cycle of backwash pumps

Raw water and transfer pumps run for hours at a fairly steady load. Backwash pumps are the opposite: a few times a day, for a few minutes each time, at high flow, and then off for hours.

This pattern has a few consequences:

• Frequent starts and stops put stress on bearings, the coupling and the motor.
• Few running hours, but many starts. A calendar PM interval rarely suits this pattern.
• Failures are noticed late. A pump that is off most of the time is rarely heard making an unusual noise.

That is why PM on these pumps is better tied to the number of washes or the actual running hours, not only to the calendar. And where possible, define a vibration measurement point on them that the operator reads during a wash.

Abnormal vibration on a backwash pump is usually only noticeable during a wash: exactly when everyone is watching the filter.

Bearings and alignment

Most failures of these pumps come from bearings and alignment. Pumps that take sudden load wear their bearings faster than expected if they are not precisely aligned. Points for maintenance:

1. Measure vibration at every PM and record the number, not just “good” or “bad”. The trend over several PMs says more than one number on its own.
2. Check alignment after every move or motor repair.
3. Lubricate according to the manufacturer’s instructions, taking the number of starts into account.
4. Keep bearings in stock. If you have two backwash pumps with the same bearings, keep at least one bearing set in the store and reserve it for PM.

When vibration is also recorded after a repair, you can see whether the repair really solved the problem. “Vibration down from 9.8 to 2.1 mm/s” is a sentence that is still worth something six months later.

Air scour blowers

Many filters are scoured with air before the water wash, to loosen the bed. Air scour blowers also run intermittently, just like the pumps. Common points:

• The inlet air filter, which clogs quickly in a dusty environment.
• The discharge check valve, which, if it fails, lets water from the filter floor flow back towards the blower.
• Bearing temperature, which can rise even in short runs.

Valves: the forgotten equipment

Every filter has several valves: inlet, filtered water outlet, wash water inlet, air inlet and wash water drain. In a plant with four or eight filters, that means dozens of valves and actuators. Their very number means they are usually summarised in the equipment list as one row, “filter valves”.

The problem is that a valve that doesn’t open and close properly ruins the whole wash cycle. An inlet valve that doesn’t close fully lets raw water into a filter that is being washed; a drain valve that sticks leaves the wash incomplete.

A suggestion: give every actuated valve a position with its own code, such as “filter 2 inlet valve”, and a QR tag on the valve itself. When the technician is at the valve, scanning the tag shows that valve’s history, and the work is recorded against it.

The link with water quality

Filter maintenance is not only about equipment. Filtered water turbidity and the head loss across each filter are the best signs of how it is performing. If one filter’s outlet turbidity is rising, or its time between washes is getting shorter, the problem may be the filter bed, the underdrain or incomplete washing.

When these readings are recorded on the same unit that the wash equipment serves, two questions can be seen side by side: “what state is filter 2 in?” and “what state is filter 2’s wash equipment in?”

An example: from vibration to history

Say the operator notices abnormal vibration on backwash pump 2 while washing filter 2. If that observation is recorded from a phone against that pump, the maintenance supervisor sees it in the review queue in the morning, accepts the request, and a work order “inspect and replace bearings, backwash pump 2” is created.

While preparing the job, the planner sees that the right bearing set is not in stock. The job is planned and released, but the shortage warning shows on it, and a supply request is recorded. Two days later the bearings arrive, are reserved for this job, and the warning clears.

The technician does the job in the filter gallery, where the signal is weak, checks the alignment and records the post-repair vibration. Pump 1 has handled the washes all along, and no filter has been taken out of the cycle.

Six months later, at this pump’s PM, the technician can compare today’s vibration with the number recorded after the repair.

One point about sequencing: if you have two backwash pumps, never take both out of service for PM at the same time, even if both are due in the same week. When the planner sees the due dates side by side, they can move one a few days earlier or later and record why, so it is clear later why it was moved.

A summary for the planner

• Tie PM on backwash pumps and blowers to how much they actually run.
• Record vibration as a number, before and after repair.
• Keep bearings and inlet air filters in stock as fast-moving parts, and reserve them for PM.
• Treat every actuated valve as its own position with a QR tag.
• Look at each filter’s turbidity and head loss readings next to the history of its wash equipment.