Reliable coagulant dosing key to successful phosphorous control

Dosing pumps help Welsh Water maintain strict phosphorous limitsIt is well documented that the appropriate control of nutrients such as phosphorous in the world’s water courses is vital for a cleaner, sustainable future. With this in mind, the European Water Framework Directive (Directive 2000/60/EC) has set an ambitious goal, demanding that rivers, lakes, coastal waters and groundwater be in a "good ecological and chemical condition" by 2027.

So, what does achieving “good chemical condition” entail? Well, as elevated levels of phosphorous lead to excessive algal growth and eutrophication of surface waters, key measures defined by the European Commission state that taking steps to protect water from the ingress of this nutrient is advantageous. 

For municipal wastewater treatment plants, this demands chemical coagulation before any discharge to water courses. Here, the addition of a coagulant such as ferric sulphate, converts the phosphate solution into insoluble phosphate compounds which can then be isolated. 

Tight restrictions

Unsurprisingly, the limit for phosphorous content (P-limit) at the discharge points of municipal water treatment plants is gradually being reduced. In Germany, the Waste Water Ordinance has published an appendix that lists P-limits on a sliding scale according to the size of the water treatment plant. 

According to the appendix, the water treatment plant in Schlüchtern (population 27,500), near Frankfurt, Germany, belongs to size category 4. 

"The P-limit for a plant of our size is 2 mg of phosphorous per litre,” explains Operations Manager Eduard Spuling. "However, as the town is upstream of the Kinzigtal reservoir, a significantly stricter discharge limit of 0.9 mg/l currently applies." 

Until recently, the Schlüchtern water treatment plant used diaphragm pumps to perform coagulant dosing operations. However, due to widely fluctuating dosage, the use of these pumps led to persistent interruption of flow. To overcome this issue, the plant turned to a 500 series cased peristaltic pump from Watson-Marlow Fluid Technology Group, a move that has resulted in dependable, high precision coagulant dosing. Furthermore, the Schlüchtern plant has been able to dispense with the need for a second pump as well as considerably reduce its maintenance requirements.

Intensive phosphorous removal

With phosphate content in the feed water at Schlüchtern averaging 5-6 mg/l, this requires a reduction of more than 80%, thus demanding intensive P-removal.

 "We have made significant investments in this area in recent years", says Spuling. "For instance, we have switched from manual dosing to automatic dosing using a real-time controller. This uses the current phosphate load to automatically calculate the necessary quantities of ferric sulphate coagulant required, so that we can reliably observe the necessary discharge values. The actual dosage is then performed using a pump."  

Until recently, the Schlüchtern used diaphragm dosing pump technology, which unfortunately caused the plant a number of problems.

"As we do our coagulant dosing in parallel [in two pools], we also had two pumps, but this led to more and more issues,” recalls Spuling. “Above anything, the pumps had problems with variation in the feed volumes of the coagulant." 

Low feed volumes

Low feed volumesDepending on the amount of dissolved phosphate in the wastewater, the quantity of required coagulant can vary hugely – from 0.4 to 60 litres per hour. The problems were primarily caused by feed volumes of less than 4 litres. 

"With such low feed volumes, the flow of the diaphragm pumps was frequently interrupted,” says Spuling. “The result was increased P-values coming from our water treatment plant."

As diaphragm pumps are generally designed to have only limited self-priming capabilities, each interruption in the flow resulted in considerable workload for staff at the plant. 

"With each interruption, we had to prime the lines with a hand pump, which took about 20 minutes of hard work,” reports plant operative Michael Comes. “There was also quite a lot of call-outs at night."

If one of the pumps was damaged, it was expensive and labour intensive to replace. In addition, diaphragm pumps have many components, such as non-return valves and springs, which can clog or corrode. 

"We had to stock a whole array of replacement parts” adds Comes. “We nearly always had a diaphragm pump in the workshop. What’s more, it was extremely unpleasant and at times dangerous to handle leaking coagulant. 

Peristaltic solution

Due to positive experiences with peristaltic pumps in a previous occupation, Spuling decided to trial a Watson-Marlow 500 series cased drive fitted with multi-channel 313 peristaltic pumpheads. This model easily allows up to six pumpheads to be fitted on a single drive. The advantage for the water treatment plant is that even with two independent flows, just one dosing pump is sufficient. At Schlüchtern, the Watson-Marlow peristaltic pumpcreates several dosing points in the respective secondary clarifying tanks. As an added benefit, the 500 series pump is completely reliable, even with flow rates as low as 0.1 l/min.

"The difference in comparison with the diaphragm pumps was noticeable from the beginning, and we’ve not had further problems with interruptions in flow,” confirms Spuling. “It is also very easy to use. In contrast to our previous pumps, we can now easily input the dosing rate directly in litres per hour. Moreover, thanks to the accuracy and repeatability of the Watson-Marlow cased pump, frequent measuring is now no longer necessary. As a result, we are now looking to dispense with all our flow metering equipment in the near future." 

The water treatment plant at Schlüchtern also benefits from the peristaltic pump's longer maintenance intervals., as the two  313 pumpheads  fitted have proved  to be extremely robust and durable. 

"Whereas on the diaphragm pumps we had to often replace parts, the 500 series cased drives and 313 pumpheads have given us trouble-free operation for three years,” states Spuling. 

P-limit control

By investing in the latest phosphorous coagulation technology, the water treatment plant has been able to significantly reduce the P-limits of its discharges. 

"We are able to declare a reduction in levels from 1.2 to 0.9 mg/l currently,” reports Spuling. "With this in mind, we will make significant savings on wastewater charges moving forward. Moreover, we can offset the investment in the control module and new dosing pump against the wastewater charges." 

According to the Waste Water Charges Act, investments by plant operators can be offset against wastewater charges for up to three years retrospectively. This is, however, on the condition that the amounts of pollutant being discharged – in this case phosphorous – are reduced by at least 20%, a figure easily achieved in Schlüchtern. For the water treatment plant and the town’s inhabitants, the improvements can therefore be implemented on a cost-neutral basis. 

“We are now planning a two point coagulation strategy, as well as the potential of implementing downstream P-coagulation,” concludes Spuling. "If that happens, we will definitely be speaking to Watson-Marlow about more peristaltic dosing pumps." 

Dosing pumps help Welsh Water maintain strict phosphorous limits

To help overcome the repeated blockage of diaphragm pumps when dosing ferric sulphate, a number of Welsh Water sites now feature Qdos peristaltic pumps from Watson-Marlow Fluid Technology Group (WMFTG). The investment is expected to achieve a quick return thanks to significant savings in maintenance associated with stripping down and cleaning the diaphragm pumps. Moreover, the Qdos pumps are helping Welsh Water meet increasingly strict phosphorus limits.

The continuous dosing of ferric sulphate at water treatment works requires the application of reliable, high-performance pumps. For Welsh Water, the traditional pump type of choice has been based on diaphragm technology. However, this has had its issues.

“Our diaphragm pumps were blocking on a repetitive basis,” explains Ronnie Swain, Process Technician at Welsh Water. “Almost every time I visited one of our sites there would be problems with the diaphragm pumps. We would start off by turning them up to achieve the same flow, but eventually we’d have little option but to ask our maintenance team to open up the pumps and remove the blockage. However, this obviously involves time and cost, and it was happening year-round. We had to carry a whole array of replacement parts in stock. It was clearly time to look for another solution.”

Welsh Water opted to install a Qdos 120 pump for dosing ferric sulphate at its Rotherwas wastewater treatment plant in Hereford. Here, the pump has been set to work operating at a flow rate of 50-60 l/h. However, Qdos pumps are reliable at flows as low as 0.1 l/min.

Such was the success of the Qdos pump (no more blockages), that two more have since been duly installed at Welsh Water’s Ross-on-Wye wastewater treatment plant – one as the duty pump and one on standby. These pumps run at a much lower flow rate of 8 l/h.

“Phosphorus is already limited to 1 or 1.25 mg/l, but the expectation within industry is that it will soon become even stricter,” says Swain. “Obviously we are dosing ferric sulphate, which means we also have to be careful that we don’t fail Environment Agency [EA] tests on iron levels. This balance means we need pumps that work reliably and correctly on a 24/7 basis.”

The EA takes samples from Welsh Water plants such as Rotherwas and Ross-on-Wye 12 times a year, checking for phosphorus, iron, ammonia, solids and more.

“We have a responsibility to ensure the delivery to our watercourses is as good as it can possibly be,” concludes Swain. “We’ve calculated our averages for 2016 and I’m pleased to report that both Rotherwas and Ross-on-Wye are comfortably within the required P-limits. Our Qdos pumps have certainly played a significant part and Welsh Water is now considering adopting this technology at further plants.”

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