The operation of a pulsator clarifier can be optimised with regards to suction lift and drop times. This can be done by considering the varying turbidity of raw water (low versus high settleable solids) as well as controlling the sludge blanket depth.
By Kimon Padayachee, WISA member A pulsator clarifier plays a crucial role in the operation of the water treatment plants, aiding in the removal of suspended solids and facilitating the clarification process. It is a simple type of up-flow tank, of which its effectiveness depends on the formation of the sludge blanket. With its hopper-bottomed tank, the pulsator clarifier allows flocculated water to flow upward through the sludge blanket in alternating cycling or pulsating flow stages of suction lift and drop times. This is done via a vacuum pump that induces fluid motion and enhances the settling of suspended solids. Pulsating flow cycle The pulsating flow cycle consists of two phases:
- suction lift phase (usually 60 cm – 100 cm)
- bed expands uniformly
The sludge blanket acts as a filter, aiding in the removal of fine particles and preventing them from being re-entrained into the clarified overflow stream. During the subsiding flow phase, the vacuum air valve opens, releasing air into the drop pipe, which creates pressure and forces the sludge blanket to drop downward. The bed settles uniformly, ensuring the separation of solids from the treated water. As a result of the pulsator cycling flow, the sludge blanket remains homogeneous throughout its depth with no stratification, facilitating continuous, effective contact between coagulated water and sludge. The duration of the total pulsation cycle is varied according to the feed flowrate, water quality, and would typically be of the order of 30 – 60 seconds. This is split by suction lift time range of 20s – 50s and a drop time range of 6 – 10 seconds. These would normally be set to achieve a solids up-flow rate during the pulse in the tank of about 7 – 8 m/h for waters containing low settleable solids (< 0.25% m/v) and 10–12 m/h for waters containing high settleable solids (> 0.25% m/v). High settleable solids
Higher settleable solids sources require longer suction lift times (40 – 60 seconds) to allow pulsators to effectively draw in the solids-laden water. This case is indicative of a higher concentration of suspended solids and longer suction lift times that are necessary since as raw water passes through, the suspended blanket seizes the particles and clarified water travels upwards to the top of the unit.
High velocity of flow through the sludge blanket creates greater resistance, high degree of particle collisions between primary flocs presents in coagulated water and secondary flocs present in the sludge blanket which results in a higher flocculation energy that leads to a consistent and stable sludge blanket formation. Flocculation energy refers to the energy required to promote the aggregation and growth of particles into larger flocs during the flocculation process. The flocculation energy process occurs under the hydraulic pulsation mechanism where there is a periodic variation of water levels within the clarifier using hydraulic pulses or surges as described above. These pulsations create a back-and-forth flow pattern within the clarifier, aiding in the flocculation process by enhancing particle collisions and floc growth. By extending the suction lift time, pulsator clarifiers have additional opportunity to thoroughly extract the suspended solids from the raw water. The increase in volume concentration of sludge blanket improves the flocculation efficiency process which occurs within sludge blanket. It is also advisable that during the occurrence of high settleable solids in the raw water, the plant should not utilise a Streaming Current Detector (SCD) analyser for coagulant dose control. An SCD analyser is commonly used in water treatment plants to measure the electrical charge or zeta potential of particles in the raw water. It assists in monitoring and controlling the coagulant dose by providing real-time response on the effectiveness of the coagulation process. However, under conditions of high settleable solids, utilising an SCD for coagulant dose control is not suitable for the following reasons:
- There are a significant number of particles present in the raw water, which interferes with the accurate measurement of SCD analyser due to sensor fouling. The suspended particles can affect the electrical conductivity and charge distribution within the analyser, leading to inaccurate readings and unreliable coagulant dose control. During the April 2022 flooding event, at a raw water turbidity of 3000 NTU, the SCD analyser provided a coagulant dose of 128 ppm whereas a jar test provided an optimum dose of 60 ppm. This contribution of increased coagulant overdosing immediately led to the formation of small compact flocs which remained suspended in the pulsator clarifier and resulted in complete cloudiness at the surface. In addition, this also caused filter blinding resulting in minimal effectiveness in removing solids and increased final water turbidity. In order for the treatment process to continue successfully, the ‘overdosed’ water at each process train was discarded to the sludge plant.
- Inadequate Charge Detection: The SCD relies on the measurement of the electrical charge carried by the particles in the raw water. However, under conditions of high turbidity, the particles may have irregular shapes, sizes, and compositions, making it difficult for the SCD to accurately detect and measure their charges. This can result in inconsistent or incorrect readings, leading to ineffective coagulant dose control. Under the above conditions, coagulant proportional dose control is recommended where the jar test optimum coagulant dose can be used as the set-point value. It is recommended to conduct cascade tests and jar tests every hour and two hour respectively under high settleable, solids raw water conditions until the raw water turbidity stabilises.
Low settleable solids Conversely, low settleable solids sources allows for shorter suction lift (20s-35s) and drop times of (6 – 8 seconds) as there are fewer solids to be lifted through the vacuum chamber. These times can be varied after measuring the pulsator clarifier overflow turbidity, a general guideline an acceptable overflow turbidity required is < 5 NTU. This will suggest that the optimum flocculation conditions are achieved within the pulsator clarifier. To optimise the flocculation process within the sludge blanket of the pulsator clarifier, it is advisable to maintain a minimum depth of 0.4 m from the top surface. This guideline serves as a reference for determining the frequency of pulsator clarifier desludging. However, it is crucial to exercise caution when adjusting the desludging timing to avoid the complete loss of the sludge blanket. In addition, it is important to ensure that the pulsator clarifier is operated at its point of fluidisation, meaning that there must be a balance between the up-flow velocity of the coagulated water and the settling velocity of the fluidised bed. Point of fluidisation occurs at the design capacity of the pulsator clarifier. Should the treatment plant be operated below the design capacity, poor clarified water (> 5 NTU) and increased solids loading occurs at the filtration plant which results in poor final water quality (> 0.5 NTU). In comparison to laminar flow type clarifiers, also referred to as tube settlers or lamella clarifiers, these units operate on the principle of laminar flow and inclined plate settling. These clarifiers utilize a series of inclined plates or tubes, called lamellas, to create a large effective settling area within a compact footprint. The influent enters the clarifier and flows through the inclined plates, causing the flow to become more laminar. The laminar flow allows for gentle and controlled movement of the liquid across the inclined plates. As the liquid flows over the plates, the suspended solids settle due to gravity and slide down the inclined surface. The settled solids collect at the bottom of the clarifier, forming a sludge layer. This sludge is periodically removed through sludge collection mechanisms. The clarified liquid rises to the top and exits the clarifier. The inclined plates in a laminar flow type clarifier provide a larger effective settling area compared to conventional settling tanks. This allows for enhanced particle settling and improved separation efficiency within a smaller footprint. The principal operation of pulsator clarifiers relies on hydraulic pulsation to induce settling and separation of solids, while laminar flow type clarifiers utilize inclined plates to create laminar flow and enhance settling efficiency. Both types of clarifiers have their advantages and are suitable for different applications depending on the specific requirements of the water treatment process.
