CASE STUDY · 12
Will a Sludge Concentration Meter Get Tangled in Hair? Measuring Point Selection and Dosing Control in Thickening and Dewatering
Chirped ultrasonic acoustic-impedance measurement in sludge thickening, digestion and dewatering dosing control
Scope: the sludge section of municipal wastewater treatment plants — thickener underflow, anaerobic digestion recirculation and dewatering machine feed; also applicable to coagulation and sedimentation in waterworks and to settled sludge in industrial wastewater plants
▍Project Snapshot / Process Conditions
Customer industry | Municipal water treatment / sludge treatment |
Region | A 300,000 t/d municipal wastewater treatment plant in eastern China |
Measured medium | Municipal sludge (thickener underflow / digester recirculation / dewatering machine feed) |
Medium temperature | Ambient to 35 ℃ (digestion section; meter rated 0 ~ 80 ℃) |
Pipe size / installation | DN100 ~ DN200; measuring spool flanged into a straight run, non-intrusive sensor, lining and materials specified for the duty |
Range / accuracy | 0 ~ 80 % concentration by weight, ± 1 % FS (published in the datasheet as ± 1 % concentration by weight); actual sludge solids are low, so range limits must be set per section and each point calibrated separately |
Field calibration | Gravimetric sampling method; on-line calibration with no shutdown; each section calibrated separately, with no coefficients borrowed between sections |
Replaces | Nucleonic (gamma) density gauge, differential pressure instruments |
Operating record | 8 months in service, 3 units running (as reported by the plant) |
1 Process Background and Measurement Challenges
▍1.1 Sludge concentration determines dosing, dewatering performance and the volume hauled away
The sludge section of a municipal wastewater treatment plant is a series of volume-reduction steps: waste sludge from primary and secondary settling (about 0.5 % ~ 1.0 % solids) is first thickened to 3 % ~ 6 %, then enters the anaerobic digester (about 35 ℃, 20 ~ 30 days retention) for stabilisation and volume reduction, and is finally dewatered into cake by a centrifuge or belt press with PAM flocculant, for incineration, landfill or beneficial use. The same type of measurement applies to floc concentration in the coagulation and sedimentation stage of waterworks and to sludge concentration in neutralisation and settling tanks in industrial wastewater plants. Concentration acts progressively along this train:
▪Thickener underflow concentration — reflects thickening efficiency and any solids carried over in the supernatant; a low value means the thickening time or the sludge withdrawal rhythm needs adjusting;
▪Digester recirculation sludge concentration — bears on gas production and mass transfer in the tank; too high a concentration impairs mixing and heat transfer;
▪Dewatering machine feed concentration — the key control point of the whole train, directly determining the PAM dose and the cake moisture: if the feed concentration fluctuates while the dose stays fixed, the result is either wasted polymer or insufficient flocculation;
▪Cake moisture — ultimately determines the volume of sludge hauled away and the burden on disposal, and is the most direct performance indicator for the sludge section.
The range convention also has to be stated first: actual solids content in the sludge section is far below the 0 ~ 80 % full range of the meter — only 0.5 % ~ 1 % before thickening and typically 3 % ~ 6 % at the dewatering feed. The correct practice in this industry is therefore not "one meter covering every section", but setting the upper and lower range limits of each measuring point to its actual range and calibrating each point separately; the strategy is set out in the questions and answers in Section 5.
▍1.2 Limitations of Existing Measurement Methods
▪Gamma (nuclear) density gauges: municipal plants are busy sites, with operating, maintenance, visiting and trainee personnel all moving through, so the burden of safely controlling a radioactive source is heavy; many regions no longer approve new sources for wastewater projects, and installed gauges face decommissioning arrangements.
▪Differential pressure / diaphragm density meters: anaerobically digested sludge contains hydrogen sulphide and organic acids, so diaphragm corrosion and zero drift occur together; organic matter adhering to the diaphragm face amplifies the error further, and cleaning is frequent.
▪Insertion-type vibrating fork / rotor instruments: sludge contains hair, fibres, textile fragments and pieces of plastic, which wrap around the fork tines or the rotor root into a mass; the plant reports an average of one shutdown cleaning per week, with the data interrupted while cleaning.
▪Sampling analysis plus dosing by experience: PAM dosage is estimated from shift experience and intermittent laboratory results, so when the feed concentration changes suddenly the dose cannot keep up — overdosing wastes polymer, while underdosing breaks up the flocs and raises cake moisture.
Figure 1 Concentration measuring points in the sludge treatment process of a municipal wastewater plant (thickener underflow / digester recirculation / dewatering machine feed)
2 The PS7000 Technical Approach
▍2.1 Measuring principle: the chirped (linear frequency modulation) acoustic-impedance method
When an ultrasonic wave travelling in a medium meets a solid-liquid interface, the echo amplitude is determined by the difference in acoustic impedance across that interface, and acoustic impedance Z = medium density ρ × sound velocity c is directly related to the density of the medium. The PS7000 uses a single self-transmitting, self-receiving sensor that continuously emits chirped (linear frequency modulation) ultrasonic pulses and acquires the echoes in real time. The wide bandwidth of the chirp signal helps the pulse penetrate bubble layers and suppresses multiple-reflection interference, and the transmitter then resolves the acoustic-impedance signature of the echo to recover the density of the medium.
The fundamental difference from the gamma-ray method is that the PS7000 contains no radioactive source of any kind. It is a non-nuclear instrument: no radiation safety licence is required, and there is no annual source inspection, no radiation-worker qualification and no source decommissioning and disposal.
▍2.2 Non-intrusive measurement: the sensor is wetted but does not protrude into the flow path
The PS7000 measuring spool is flanged into a straight run of the process line and the ultrasonic sensor is mounted on the spool. This is the key structural difference from conventional insertion-type instruments, and the point most often misunderstood during selection, so it is set out here exactly as it is worded in the Operating Manual:
▪Non-intrusive (non-intrusive is not the same as non-contact): the sensor must be wetted by the medium in order to work, but it does not protrude into the flow path — there is nothing projecting into the bore. The manual is explicit on this point: for horizontal installation with the sensor on top of the pipe, a full pipe must be guaranteed, otherwise the sensor loses contact with the liquid and the measurement fails; for remote-mount horizontal installation the sensor should be placed on the underside of the pipe so that it stays wetted.
▪There is therefore none of the leading-edge erosion, build-up at the probe root or downstream flow disturbance associated with insertion probes, and no risk of blocking the line.
▪The acoustic window of the sensor is sapphire: a fine surface finish, wear-resistant and corrosion-resistant. The measuring spool can be supplied with a lining (PTFE / rubber / ceramic) to suit the duty.
▪Heavily scaling media should still be included in routine inspection — the manual lists "scaling on the sensor face" as one of the items to check when calibration fails, the remedy being to clean the sensor and recalibrate.
▪Applicability limits: bubbles in the medium are permitted; the medium should not, however, contain large quantities of suspended or settled coarse particles.
Figure 2 Sectional view of the PS7000 installation — the structural difference from conventional insertion types (schematic drawing of the principle, not a record of measured data)
▍2.3 Key Technical Specifications
Item | Specification | Item | Specification |
Measuring principle | Chirped ultrasonic acoustic-impedance method | Pipe diameter | DN50 ~ DN1000 |
Measuring range | 0 ~ 80 % concentration by weight / calibratable on the actual medium | Installation | Integral / remote-mount, flanged into a straight run |
Accuracy | ± 1 % FS (the published datasheet figure is ± 1 % concentration by weight) | Dimensions | 288 (W) × 190 (D) × 95 (H) mm |
Power supply | Standard AC220V (175 ~ 285V, 50/60Hz); DC24V optional | Weight | Approx. 2.5 kg |
Power consumption | ≤ 1 W | Display | OLED screen (bilingual Chinese / English menu) |
Ambient temperature | -30 ℃ ~ 60 ℃ | Analogue output | 4-20 mA, 2 channels |
Medium temperature | 0 ℃ ~ 80 ℃ | Digital interface | RS485 / Modbus RTU, approx. 500 m |
Humidity range | ≤ 98 %RH | Ingress protection | IP65 |
Data storage | 2000 real-time records + 400 daily records (customisable) | Remote transmission | Optional 4G module |
Field calibration | Gravimetric sampling method, performed online with no shutdown | Custom options | Lining, range, explosion protection, high temperature and similar options built to the duty |
※ Actual solids content in the sludge section is far below the 0 ~ 80 % full range (about 0.5 % ~ 1 % before thickening, about 3 % ~ 6 % at the dewatering feed), so the upper and lower range limits should be set for each measuring point according to its actual range and each point calibrated separately. ± 1 % FS (published in the datasheet as ± 1 % concentration by weight) is a general figure; at a pre-thickening point where solids are only 0.5 % ~ 1 %, that deviation is of the same order as the measured quantity itself, so such points are best used for trend monitoring and relative change control and should not serve as the basis for performance accounting.
▍2.4 Selection and Installation Requirements (important)
The following conditions directly determine the stability of the measurement in the field. We recommend confirming them while the measuring point is still being designed; doing so avoids the great majority of post-commissioning disputes over deviation.
▪Vertical installation with upward flow is preferred. For horizontal installation, confirm that the medium is not stratified and that the measuring point is representative.
▪Flow velocity > 1 m/s; the higher the density of the solid phase, the higher the velocity required. Avoid low-velocity or stagnant duties, in which particle settling disturbs accuracy and stability.
▪Straight run requirement: the installation position should be well clear of pumps, valves, bends and other local resistances; for horizontal installation, 10D upstream / 5D downstream of straight run must be provided (5D upstream / 3D downstream as an absolute minimum).
▪Keep the sensor wetted: for integral horizontal installation with the sensor on top of the pipe, the pipe must run full; for remote-mount horizontal installation the sensor should be placed on the underside of the pipe. Where the pipe does not run full, the remote-mount version is preferred.
▪Avoid points where free gas bubbles accumulate. Aeration, twin-pump suction, negative-pressure inlets and similar conditions introduce large quantities of free gas bubbles; the bubbles raise the echo energy and bias the reading high. This is an inherent limitation of the acoustic method and must be handled by the choice of measuring point, not by compensating in calibration.
▪The comparison sampling point should be as close as possible to the measuring point. A sampling point far from the measuring point introduces transport lag, so the laboratory reference value and the displayed reading no longer refer to the same moment, and the difference is easily mistaken for instrument error.
▪The measuring point should be downstream of screening and grit removal: the applicability boundary in the manual is that "the medium should not contain large quantities of large suspended or settled particles". Grit, plastic lumps and coarse inorganic debris both scatter the echo and may settle inside the measuring spool, so we do not recommend placing the point on primary sludge or on a line upstream of grit removal; the dewatering feed pump discharge is the most dependable measuring point in this industry.
▪Sludge pump pulsation and low velocity must be checked together: sludge is usually moved by progressive cavity or plunger pumps, whose discharge pressure and flow pulsate, so the measuring point should be well away from the pump outlet on an adequate straight run, with the measurement damping raised somewhat if necessary. Velocities in sludge lines are often low and must be checked against the > 1 m/s requirement; where the velocity is low, a vertical upward run is the first choice.
▍2.5 Field calibration: the gravimetric sampling method, performed online
Calibration brings the displayed value into agreement with the true density of the medium: several pairs of "instrument reading + corresponding laboratory reference value" are collected and the instrument automatically regresses the correction coefficients. The whole procedure requires no shutdown and can be carried out online provided the process is steady and the sensor is clean.
Process conditions to be confirmed, item by item, before calibration:
Full pipe | The sensor must be completely immersed in the liquid |
Steady flow | Variation within ± 5 % |
No significant bubbles | No venting operation should be in progress in the line |
Steady temperature | A change in slurry temperature affects the measurement signal |
Clean sensor | No significant deposit or scaling; rinse with clean water if necessary |
The most common situation in the field is that only the process medium is available and clean water cannot be introduced temporarily. In that case two consecutive acquisitions are made on the same medium (about 2 minutes each, about 4 minutes in total): the two measured points plus the virtual zero point added automatically by the system form three regression points, and the instrument uses the goodness of fit r² to cross-check the consistency of the two signals. If bubbles, a step change in flow or an abnormal valve movement during acquisition make the two data sets inconsistent, the system rejects the calibration outright rather than returning an incorrect coefficient. This is the key mechanism that prevents a calibration that looks successful but is in fact wrong.
Recommended calibration interval: calibrate a newly installed instrument once within the first 12 ~ 24 hours of operation; once operation is stable, verify every 6 ~ 12 months; verify every 1 ~ 3 months at measuring points with demanding accuracy requirements; recalibrate immediately after changing the sensor or the medium.
3 Before / After Comparison
Aspect | Original gamma-ray / differential pressure solution | PS7000 solution |
Radiation compliance | The radioactive source requires a licence, annual inspection and decommissioning and disposal, and control is demanding on a busy plant site | A non-nuclear instrument, with no radiation-related approvals |
Wrapping by debris | Inserted fork tines / rotor become wrapped in hair and fibre, requiring a weekly shutdown for cleaning | Nothing protrudes into the flow path, so there is no root for wrapping; the medium should nonetheless not contain large quantities of large particles, so the measuring point must be downstream of screening and grit removal |
Corrosion and drift | The diaphragm corrodes in sulphur-bearing digested sludge and the zero drifts continuously | The non-intrusive sensor does not protrude into the flow path; lining and materials are specified for the duty, and the sensor face should still be covered by routine inspection |
Data continuity | Shift-based sampling and laboratory analysis, with data intervals measured in hours | Continuous on-line output, usable as the feedforward signal for PAM dosing |
Range matching | A fixed range, with insufficient resolution in the low-solids sections | Upper and lower range limits can be set per section, with thickening, digestion and dewatering calibrated separately |
System integration | Usually a single analogue output only | 4-20 mA × 2 + MODBUS-RTU into the PLC / central control system, optional 4G for connection to a smart water platform |
4 Field Verification and Operating Record
On the upgrade project of a 300,000 t/d municipal wastewater treatment plant in eastern China, three PS7000 units replaced the existing nucleonic density gauges and differential pressure instruments at the thickener underflow outlet, on the digester recirculation line and on the dewatering machine feed line. The dewatering feed is the key measuring point, its concentration signal being fed into the PAM dosing control loop as a feedforward term; all three points were calibrated separately on the sludge of their own section.
After 8 months in service the plant reports the following operating statistics: PAM flocculant consumption down by about 18 % against the same period before the retrofit, and average cake moisture down from 82.3 % to 78.5 %. These figures are taken from the plant's own operating reports and have not been verified by a third party; sludge characteristics, dewatering machine type and polymer grade differ considerably between plants, so your own comparison trials should be the reference. Field comparison used the gravimetric sampling method, with the sampling point close to the measuring spool to reduce the method error caused by transport lag.
A Note on Comparison Methodology The gravimetric sampling method itself carries method error: the physical distance between the sampling point and the measuring point introduces transport lag, and the representativeness of the sampling operation together with the weighing and drying accuracy in the laboratory also contribute to the deviation. We therefore avoid expressions such as "in complete agreement", and recommend instead that acceptance be based on the mean deviation and the trend consistency across several consecutive comparison sets, with the sampling rules stated explicitly in an annex to the contract. Pisonics can provide a standard field comparison and acceptance procedure. |
▍Verifiable Benefits for the Customer
▪There is no longer a radioactive source on site, which removes the licence application, annual inspection, personnel qualification and source decommissioning and disposal.
▪The dewatering feed point no longer needs a weekly shutdown to clear hair and fibre, and the concentration data are continuously available.
▪With feed concentration acting as a feedforward signal for PAM dosing, there is less of the lagging adjustment in which "the cake is seen to deteriorate and only then is the dose changed".
▪With the concentrations from thickening, digestion and dewatering all in the central control system, the sludge section now has continuous material data, which makes it easier to reconcile haulage volumes and disposal records.
5 Frequently Asked Questions
Q1 Sludge is full of hair, fibres and plastic fragments — will the sensor get tangled or blocked?
A Freedom from wrapping is indeed an advantage of this structure: the sensor does not protrude into the flow path, nothing projects into the flow, and there is therefore no root for fibre to wrap around — an entirely different failure mechanism from insertion-type forks and rotors. Please note the other boundary at the same time, however: the manual states explicitly that "the medium should not contain large quantities of large suspended or settled particles". Grit, plastic lumps and coarse inorganic debris scatter the echo and may settle inside the spool, and duties of that kind are outside the application range. The measuring point should therefore be downstream of screening and grit removal, away from primary sludge and other lines with a high debris load. In addition, organic matter in sludge readily forms a deposit on the sensor face; the manual lists "scaling on the sensor face" as one of the items to check when calibration fails, so it should be covered by routine inspection, with re-calibration after cleaning.
Q2 Waste sludge is only 0.5 % ~ 1 % while the meter range is 0 ~ 80 % — can it be measured? How is each section calibrated?
A The answer differs by measuring point. Dewatering feed (3 % ~ 6 %) and digester recirculation sludge are the main measuring points in this industry: the concentration is moderate and the velocity assured, and once calibrated on site with the plant's own sludge the reading can be used for control. Waste sludge before thickening (0.5 % ~ 1 %) calls for realism: ± 1 % FS (published in the datasheet as ± 1 % concentration by weight) is a general figure, and at that concentration level it is of the same order as the measured quantity itself, so such a point is better treated as a reference for trend monitoring and relative change than as performance accounting. Three calibration rules apply. First, set the upper and lower range limits at each point according to its actual range, instead of leaving every point on the full 0 ~ 80 % range. Second, calibrate each section separately: thickened, digested and dewatering sludge differ in organic content and particle structure, and their coefficients cannot be borrowed from one another. Third, re-check after any marked change in influent quality, sludge age or season — the manual recommends a first calibration within 12 ~ 24 hours of installation and re-checks every 6 ~ 12 months once operation is stable.
Q3 Can it be used directly for fully automatic PAM dosing?
A We recommend using it as a feedforward signal, not as the sole input to a fully automatic closed loop. Feed concentration is only one of the factors affecting flocculation; sludge age, organic fraction, temperature, PAM make-up concentration and ageing time matter just as much, and tying the dose entirely to a single concentration signal will go wrong as soon as the sludge characteristics change. The more robust engineering practice is to use the concentration signal as the feedforward basis for the nominal dose, correct it by feedback from cake moisture or filtrate turbidity, and keep manual override available to the operator. It should also be stated plainly that the manual gives no response time figure and we make no claims such as "response in seconds" — the meter provides two adjustable parameters, measurement damping and measurement interval, and the time constants of the dosing loop should be tuned on site to the actual process.
Q4 Does the sensor actually touch the medium? Will it be worn away?
A It does touch it. The manual's formal wording is "non-intrusive", not "non-contact" — the sensor has to be wetted by the medium to work (with horizontal installation and the sensor on top, the pipe must run full or the measurement fails), but it does not protrude into the flow path and there is nothing projecting into the bore, so the main wear mechanism of insertion probes, the leading edge being continuously scoured by fast-moving solids, does not exist. Please note that some material in circulation describes this arrangement as "non-contact", which is not accurate. Within the Pisonics range, the products that genuinely do not touch the medium are the PS7010 clamp-on type (acoustic attenuation method) and the nuclear gamma-ray density gauge.
Q5 How is it calibrated in the field? Is a shutdown needed? How often?
A By the gravimetric sampling method, carried out from the handheld remote menu, with no shutdown — calibration can be done online provided the pipe is full, flow variation is within ± 5 %, no venting is in progress, the temperature is steady and the sensor is clean. Where only one medium is available on site, two consecutive acquisitions are made on that medium (about 4 minutes in total) and the system cross-checks the consistency of the two signals with r², rejecting the calibration outright if they disagree. Interval: first calibration within 12 ~ 24 hours of installation, verification every 6 ~ 12 months once stable, every 1 ~ 3 months for high-accuracy points, and immediate recalibration after changing the sensor or the medium.
Q6 Should the PS7000 or another model be selected for a given measuring point?
A The PS7000 (ultrasonic acoustic impedance) is aimed at the density of bubble-laden, high-solids slurries. If the pipe cannot be tapped, or an ageing line is being retrofitted, choose the PS7010 clamp-on type (acoustic attenuation); for concentration analysis of clean liquids choose the PS7020 (sound velocity); for scaling-prone acids and alkalis in chemical service choose the PS7400 (vibrating fork); for large-bore mains above DN200 the PS7300 (differential pressure) is an option; for custody-transfer accuracy choose the PS7200 (Coriolis); for multi-component concentration analysis choose the PS7100 / 7110 (spectral / refractometric); for aqueous suspensions the PS7600 (microwave) is available. If the selection is not clear-cut, send us the process data and we will work it out for you.
About Pisonics
Xi'an Pisonics Information Technology Co., Ltd. (PISONICS) specialises in ultrasonic inline measurement technology, with products covering slurry density, solution concentration, spectral concentration, level, flow and other industrial process parameters. The PS7000 Series Ultrasonic Slurry Density Meter is the company's own non-nuclear inline slurry density monitor, and is in service in power plant desulphurisation, non-ferrous and ferrous mineral processing, coal preparation, salt lake chemicals, dredging, municipal sludge and other industries.
Ultrasonic · Density · Concentration · Inline Measurement Specialists
Company | Xi'an Pisonics Information Technology Co., Ltd.PISONICS |
Address | Room 15B016, Block A, Olympic Building, North Chang'an Road, Beilin District, Xi'an, Shaanxi, China |
Tel | +86 159-0293-2017 |
info@pisonics.com | |
Web | Chinese site www.pisonics.cn | English site www.pisonics.com |
※The process data and operating records in this case study are compiled from an actual project; Figures 1 and 2 are schematic diagrams of the principle and the layout, not curves recorded from measurement. The technical specifications of the instrument and its installation and calibration requirements are as stated in the Pisonics Ultrasonic Density Meter Operating Manual supplied with the goods, and are subject to change without notice.
