Technical field of the invention

The invention relates to a method for the removal of supernatant from media on a filter, where flocculant and/or coagulants are added to a media to enhance the drainage rate of supernatant, leaving the media at least partially dewatered, during which the surface topography of the media is measured and the amount of flocculant and/or coagulant to be added is a function of the topography of the media above the filter within a region of interest, or regions of interest.

The method is performed with a device for dewatering media containing a flocculant and/or coagulant feed device upstream of a filter, where a device, particularly an optical time of flight sensor and/or optical triangulation sensor, is provided downstream of an inflow area in order to digitise the surface topography of the media optically and is connected via a system of control to the flocculant and/or coagulant feed device, in order to control the flocculant and/or coagulant dosage added.

Prior art

Methods for separating media from supernatant are known from the state of the art, where the media such as slurry such as sewage biosolids, fibrous media or mineral slurries are dewatered with the addition of flocculant and/or coagulant. Flocculant and/or coagulant is applied to the media first, in order to cause the solid media portion of the media to destabilise, coagulate and flocculate, after which the suspended flocculated media ‘flocs’ and supernatant suspension is applied to a filter, a gravity drainage deck for example, so that the supernatant portion of the media drains away free of solids, while the concentrated solid portion remains on the filter surface.

Addition of flocculant and/or coagulant as described in the prior art has been controlled either by manual intervention or automation (reference is made to US4105558, US5380440, US5961827, US2007/0090060, US2009/0230033, US20170044034).

Technical problem to be solved

The flocculant and/or coagulant dosage is a critical parameter for the efficacy of the dewatering or screening process. An optimal, minimal dosage achieves the lowest possible moisture content. Overdosing will entrain more moisture within the solids, whilst increasing the concentration of flocculant chemicals in the retentate. This over flocculated state provides a stable plateau on which unsupervised operation can occur in a continuous manner in spite of fluctuating media solids concentrations. Underdosing leads to a rapid increase in the retentate moisture content resulting in filter table flooding. This is not conducive to reliable long-term operation.

Traditionally the dosage rate is adjusted manually by the filter operator based on visual inspection. The visual inspection is subject to the empirical judgement of each individual operator potentially leading to different interpretations and thus dosages. Time constraints aside, it is difficult for an operator to make accurate judgements frequently enough to optimise the filter continuously. Consequently, the objective of the operator is to achieve stable filtration over many hours of operation, in between observations. To reduce the likelihood of underdosing events, operators typically favour high flocculant consumption as the dewatering is more stable.

Automation efforts in prior art used indirect correlations between observable reflected light from a filter or retentate media surface and based on correlations between the reflected light from a region and calibrated moisture or rheological characteristics, control the dosage of chemical additive. Examples of interferents include fugitive and secondary light sources, failure to clean the belt filter and observing an unclean section of belt filter. All skew the sensor specificity and selectivity in the prior art methods, hampering the ability for the observation to appropriately automate the process in their respective regions of interest. The volume of media under observation and the moisture content may not, under a variety of conditions, provide an accurate correlation to the surface area of exposed filter when viewed from above, especially in a system where the inflow solids concentration changes and, the rheological behaviour of these solids varies.

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art as detailed below.

Fugitive/Secondary light sources

Issue

Prior work requires the filter/wire/screen to be clean and always displays the same colour to the camera. The colour received by the camera is the product of two components:

1 . The controlled spectrum and intensity of artificial illumination of the target area; and

2. The intensity of the reflection of the illuminating light at non-absorbed frequencies back to the camera.

The illumination must therefore always be controlled across a broad spectrum of frequencies to ensure that the controlled reflection to the digital camera will be consistently observed and processed by the system, without distortion or inaccuracies that prevent appropriate chemical dosage.

Solution and improvement

With the optical time of flight sensor and/or optical triangulation sensor approach, the illumination is not continuous and is controlled by designating only the coordinate to be observed, with multiple short duration bursts of light, during an observation period. This saturates that target with an intensity that prevents reflections of fugitive light sources from skewing the measurement technique. The colour of the designated target is not measured, only the angle/time of flight of reflected light is measured to determine the illuminated target height.

Filter cleaning and observation of unclean regions of interest

Issue

The digital optical camera pixel counting method can calculate the filter surface area covered by media as an inverse of the area of filter identified as not being covered by media on an area-balance colour contrast/discrimination basis. The method cannot determine the depth of the media above the filter within a region of interest, or the volume of media in a region of interest.

The use of gravity drainage deck ploughs to improve dewatering increases the risk of filter belt tears. As such plough design and height is carefully managed and sludge/media can at times move around or under ploughs, presenting in the furrow with sufficient depth to shield the filter from illumination and therefore prevent the camera from detecting the filter belt, resulting in the control philosophy identifying that the surface is subject to underdosing and increasing chemical dosage.

A desired moisture content can only be achieved with the correct dosage because both too much or too little flocculant have a detrimental impact on the efficiency of the dewatering or concentrating process. Where the filter is not sufficiently clean, the pixel counting method dosage will disproportionally favour overdosing as the rheological properties of the sludge cannot be observed due to the presence of unclean, obstructed filter.

Solution and improvement

In contrast the optical time of flight sensor and/or optical triangulation sensor approach allows the volume of media to be measured. Automated systematic adjustments to the chemical dose solicit a volume response from the media, allowing the minimum dosage required to achieve the minimum volume to be known and continuously assessed and therefore media moisture content minimised. The chemical dosing can be controlled to maximise the removal of supernatant volume reaching the end of the gravity drainage zone of a belt press, whilst simultaneously controlling dosage on a dosage volume per media volume basis. In this way, fluctuations in the media volume resulting from process or media changes are always subjected to the optimisation routine for automated chemical volume optimisation and it is understood that for an increased volume of media at its minimum moisture content, a larger volume of chemical must be added to achieve that minimum moisture content.

As such it is recognised that direct measurement and control of supernatant volumes removed from media, particularly when the objective of the coagulant and/or flocculant dosing is for this express purpose, is an improvement over the measurement of rheological behaviour.

Description of the invention

More precisely the invention concerns a method for dewatering, a media, said method comprising: a- Flocculating and/or coagulating the media, b- Deposing said flocculated and/or coagulated media on a filter, c- Performing a surface topography of the media using a laser time of flight sensor and/or an optical triangulation sensor to measure the surface height of the filter along with the media, d- Adjusting the amount of flocculant and/or coagulant to be added in the media according to the topography of the media.

For step a, media is flocculated and /or coagulated using natural or synthetic flocculants and/or coagulants.

Flocculants can be selected in the list: guar gums, chitosan, alginates, water soluble polymers obtained with non-ionic and/or anionic and/or cationic water-soluble monomers.

Non-ionic monomers are preferably selected from the group comprising acrylamide; methacrylamide; N-mono derivatives of acrylamide; N-mono derivatives of methacrylamide; N,N derivatives of acrylamide; N,N derivatives of methacrylamide; acrylic esters; and methacrylic esters. The most preferred non-ionic monomer is acrylamide.

Anionic monomers are preferably selected from the group comprising monomers having a carboxylic function and salts thereof; monomers having a sulfonic acid function and salts thereof; monomers having a phosphonic acid function and salts thereof. They include for instance acrylic acid, acrylamide tertio butyl sulfonic acid, methacrylic acid, maleic acid, itaconic acid; and hemi esters thereof. The most preferred anionic monomers are acrylic acid, acrylamide tertio butyl sulfonic acid (ATBS), and salts thereof. Generally, salts are alkaline salts, alkaline earth salts or ammonium salts.

Cationic monomers are preferably selected from the group comprising dimethylaminoethyl acrylate (DMAEA) quaternized or salified; dimethylaminoethyl methacrylate (DMAEMA) quaternized or salified; diallyldimethyl ammonium chloride (DADMAC); acrylamidopropyltrimethylammonium chloride (APTAC); methacrylamidopropyltrimethylammonium chloride (MAPTAC).

Coagulants can be selected in the list: poly(diallyldimethylammonium chloride), polymers obtained by reaction of dimethylamine and epichlorohydrin, ferric chloride, poly(aluminium chloride).

Preferably, the method is an inline method wherein the filter is moving and the laser time of flight sensor and/or the optical triangulation sensor are fixed above the filter on which the flocculated and/or coagulated media is deposed.

Advantageously, step d) of the method is carried out in a region of interest of the media, or multiple regions of interest of the media on a weighted basis.

The media coagulated and/or flocculated by the method of the invention is preferably a slurry, a sludge, or a pulp of any municipal, mineral, or fibrous origin.

Advantageously, the filter for step c) of the method is a continuous wire, belt, filter table, filter cloth, belt filter press or gravity drainage deck

In a preferred embodiment, the filter surface and or media surface topography are scanned, and the media topography is measured and recorded to a database as a two-dimensional coordinate dataset as a x, y coordinate system at any scan frequency or baud rate.

In another preferred embodiment the filter surface and or media surface topography are scanned, and the media topography is measured and recorded to a database as a three- dimensional coordinate dataset as a x, y, z coordinate system at any scan frequency or baud rate.

Advantageously, coordinate database is accessed by a programmed routine, to classify the topography of the media at coordinates in a region of interest and/or multiple regions of interest against a set point media topography and/or a matrix of coordinate set point heights thereby identifying the height at a coordinate as the same, or in some measure of error with respect to that set point or matrix of set points thereof. Preferably, the coordinate point cloud database is accessed by a programmed routine, selecting data from within regions of interest to integrate the cross-sectional surface area(s) and/or calculate the volume of the media within the region(s) of interest, identifying that cross- sectional surface area and/or volume, as the same, or in some measure of error with respect to a set point.

Advantageously, with the method of the invention, a setpoint is obtained or recorded for export, modification, importation, and/or later use, by scanning a filter and recording the media topography for reference purposes.

Examples

Optical time of flight and/ or optical triangulation

The optical time of flight sensor and/or optical triangulation sensor illumination is not continuous and is controlled by designating only the coordinate to be observed, with multiple short duration bursts of light, during an observation period. This saturates that target with an intensity that prevents reflections of fugitive light sources from skewing the measurement technique. The colour of the designated target is not measured, only the angle/time of flight of reflected light is measured to determine the illuminated target height. This solves the technical issues related to colour discrimination brought about by fugitive light, media colour changes, filter colour changes, deficient belt cleaning and media ploughing.

Furthermore, the technique of quantifying the cross-sectional surface area and/or volume of media on the filter, allows the measurement and control of supernatant volumes removed from media. The key objective of the coagulant and/or flocculant dosing is for this express purpose. As such, the method described is an improvement over the measurement of un-utilised filter as a quantification of rheological behaviour, where rheological behaviour is altered by many physio-chemical relationships, rather than just the volume of supernatant in the retentate media.

Cross-sectional media surface area

From the digitised information, the cross-sectional surface area of the media above the filter can be measured for comparison to a set point and the chemical dosage optimised.

Media distribution

From this digitised information, the distribution of the media above the filter can be measured for comparison to a set point and the chemical dosage optimised to control the wetted surface area of the filter cloth. Ploughed media furrow geometry

On the gravity drainage deck portion of belt filter presses there is typically multiple series of ploughs to turn the sludge over in order to maximise the release of free water. Immediately behind these ploughs a furrow is present.

From this digitised information, the geometry of the furrows is measured, and the chemical dosage optimised to maintain set point furrow width or a ratio of furrow area to total filter belt media area.

Region of interest geometric features

From this digitised information, the geometry of a selected region of interest is measured and the chemical dosage optimised, or alarms triggered to alert operators of a specific condition, such as but not limited to, filter hole/tear detection, low/high volume alarms, belt filter tracking issues, no flow/no movement and other persistent abnormal condition monitoring alarms.

1 ^st trial: Belt filter press unit flocculant dosage is not controlled/ governed via any mechanism and is set at a fixed pump speed or fixed flow rate by an experienced operator. Observation:

• the retentate media has a greater volume due to increased moisture content

• the retentate media topography has a lower standard deviation due to increased moisture content

• the belt filter press is not prone to instability when slurry is consistent, but at the cost of increased flocculant usage.

• the belt filter press is more prone to instability when slurry is inconsistent, and more than the prescribed dosage is justified.

2 ^nd trial: By using method of the invention coagulant/flocculant dosages less than those prescribed by an experienced operator were observed.

Observations:

• the retentate media has a reduced volume due to lower moisture content at the determined optimal dosage.

• the retentate media topography has a higher standard deviation due to lower moisture content at the determined optimal dosage

• the belt filter press is not prone to instability when slurry is consistent, and the cost of flocculant usage is less than that of fixed pump speed and fixed flow rate control by an experienced operator. • the belt filter press is less prone to instability when slurry flow is inconsistent over multi hour time scales, as the inconsistency is observed, and the optimal dosage determined and applied.