Sedimentation is the most important and most widely used unit process in wastewater treatment plants. Through gravity, the sludge particles settle to the bottom of the sedimentation tank while the wastewater is detained. Under normal con¬ ditions, the water flows continuously through the sedimentation basin, and the settled sludge is simultaneously collected and moved toward a sludge pocket on the bottom of the tank under the inlet section. The sludge is periodically pumped from this pocket for further treatment and dewateπ'ng outside the settling tank.

In most cases, settl ng tanks are built as open, rectangular tanks using reinforced concrete, with a depth to width to length ratio of approximately 1 :1;5:10. Water depth is normally 2 to 3 metres and the detention time of the water is between 1 and 3 hours, depending on local regulations.

The sludge-scraping mechanism in the rectangular settling tanks is generally one of two different types: the continuous chain type and the traversing wagon type.

The continuous chain scraping mechanism consists of a Large number of scraper blades mounted on continuous chains with fixed wheels that are powered by a gear motor mounted above the water level. The wheels are placed on both sides of the tank and move the chains and blades so that they scrape the bottom of the tank and slowly move the settled sludge from the outlet end of the tank toward the pocket at the inlet end. The wheels move on a continuous basis under water, moving some blades toward the sludge pocket whilst returning others to

the starting point for a new round of scraping. This mechanism requires a large number of moving parts, nearly all of which are under water and thereby develop corrosion and wear problems with resulting need for maintenance and replacements.

The traversing wagon type of sludge-scraping mechanism consists of a single, wide blade which covers the enti re width of the bottom of the settling tank. The blade is hung from a bridge which spans the width of the tank and which is supported by an undercarriage on wheels on each side of the tank. The wheels run on rai ls which are placed on the top of the side walls of the tank, and which run nearly the entire length above the water surface. The wagon begins its cycle with the large scrap¬ er blade touching the bottom of the tank, and moves slowly from the outlet end toward the inlet end where the sludge is scraped toward the sludge pocket. Before the undercarriages and bridge are returned to the outlet end for the next scraping cycle, the scraper blade is raised to an angled position over the tank bottom, so that it wi ll not s r up the settling sludge along the bottom whi le it returns to the starting point.

The traversing wagon is normally powered by an electric or hydraulic motor at each end wh ch supplies power to the under¬ carriages either directly through the rai ls or through an insu¬ lated electric cable which is rolled up and then out again to the unit as t moves back and forth above the tank.

The raising and lowering of the scraping blade is normally performed by a mechanism using a separate electric motor.

Since the traversing bridge, with its large scraper blade and undercarriages on wheels, is an extremely heavy construction there is a great deal of strain and wear on the electric

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power units as well as the many motors and moving parts in the scraper-posit oning mechanism, with resulting ma ntenance and replacement problems.

In many of today' s modern sewage- reatment plants, the trend is toward enclosing tanks, basins and other equipment for sewage and wastewater treatment, primari ly to avoid odours, sewage droplets and bacteria aerosol being released in the immediate working area as well as in the environs of the treatment plant. The concept of an enclosed tank allows collection and effective treatment of the foul ai r.

Both of the above-mentioned sludge-scraping mechanisms, the continuous chain and the traversing wagon, are in practice incompatible with the enclosed tank concept.

A sludge-scraping mechanism for an enclosed tank concept must be simple enough in construction to avoid the traditional main¬ tenance problems. The number of moving parts should be kept to a minimum, and all moving wheels, chains, joints or rai ls should be necessary.

With the invention described below, the scraper mechanism for sludge treatment in an enclosed settling tank, these ideal conditions are met. Detai ls of operation wi ll be described below, with numbers which refer to the enclosed drawings (Figures 1 and 2) :

Sewage to be treated by sedi entation is led through an inlet pipe (2) through an enclosed settling tank (1) past the inlet velocity arrester (3) to the outlet section which has a number of fixed, submerged outlet pipes (5) and channels (6) perforated with holes (7), and leaves the settling tank through an outlet pipe (9) after flowing over a wei r (8) which controls water leve

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In order to keep the bottom of the tank free from settled sludge a moving scraper blade ⁽ 12) made of a semi-rigid material is used. If the settling tank is cyl ndrical, as shown in Figure 1, the scraper blade (12) must reach high enough along the sides of the tank so that its angle θ i s greater than the angle pro¬ fi le of the settling sludge particles as they slide downwards - normally 60°. If the settling tank s rectangular, the scraper will have the same width and profile as the bottom and sides of the tank.

The scraper blade (12) is rigidly connected by a support prof le

⁽ 13 ⁾ to a wagon ⁽ 15 ⁾ with four wheels (16) wh ch roll with mini mal friction in rails constructed from two parallel u-profiles

(14) positioned with the openings on their sides facing each other, at the inside top of the tank above the water surface. The sludge-scraping mechanism consisting of the wagon (15), the support profile ⁽ 13) and scraper blade ⁽ 12 ⁾ is moved horizon¬ tally, pulled by a continuous wire ⁽ 17) running over two wheels (18 ⁾ which turn at each end of the settling tank (1). One of the wheels (18) is powered by an electric motor (19) which allows for alternating the direction of rotation. The wire (17) is maintained in a tight position through use of a spring (20) which connects the ends of the wire (17) to form a continuous loop. To illustrate the principle in Figure 1, the turning wheel (18) and wire (17) are mounted in a horizontal position inside the tank above the water level.

During the normal slow sludge-scraping motion of the scraper- blade (12) while t moves from the outlet end toward the sludge- collecting pocket (10) under the inlet section of the settling tank, the scraper-blade touches the tank bottom ⁽ 1 ⁾ and pushes the settled sludge forward with the support profile ⁽ 13) in a vertical position as shown n Position 1 in Figure 1. All 4

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wheels (1ό) of the traversing wagon (15) are positioned be¬ tween the flanges of the u-profiles (14) as shown in Detail A in Figure 2.

When the scraper blade (12 ⁾ has completed its cycle toward the inlet end of the settling tank (1) and scraped the collected sludge into the sludge pocket (10), the blade (12 ⁾ must be raised into an elevated position so that sludge which has set¬ tled on the tank bottom (1) will not be disturbed during the return movement. To get the scraper blade (12 ⁾ and support profile (13) into this position, in which the support profile

(13) forms the ang le 5 shown in Position 2 in Figure 1, the two front wheels (16) of the transversing wagon (15) roll over and press down a flexible steel blade (22A) just before the forward motion toward the inlet section is arrested by a stopper (21A).

The re-positioning of the sc raper-=-b lade (12 ⁾ is performed auto¬ matically at the rounded section of the upper flange (14A) at the inlet side end section of the u-profiles (14). At this point the flexible steel blade (22A) replaces the lower flange

(14) of the u-profile (14). When the scraper wagon (15) is pulled toward the inlet end, the two front wheels (16) wi ll be forced through the opening between the upper flange (14A) and the steel blade ⁽ 22A ⁾ until the wheels (16) slip through the gap and are now in a new position underneath the blade (22A) and the lower flange o-f the u-profiles (14), at which point a stopper ⁽ 21A) arrests the further forward motion of the wagon ⁽ 15 ⁾ . The stopping of the wagon (15) may itself trigger the electric motor (19) to change its direction of rotat on in order to return the wagon (15) to the outlet end of the tank (1), or this change in direction of motor rotation may also be effected by fixed electrical contacts (23) triggered by adjust¬ able contacts attached to the wire (17).

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Before reaching its original position at the outlet end of the tank (1), the front wheels (16) of the wagon (15) are now placed under the lower flanges of the u-profiles (14). When the wagon ⁽ 15 ⁾ reaches the outlet end of the tank (1) this set of wheels ⁽ 16 ⁾ will be guided vertically upward through the opening between the special sloping sect on of the lower flange of the u-profiles (14). When the wagon (15) reaches the outlet end of the tank (1) this set of wheels (16) will be guided vertically upward through the opening between the special slop¬ ing section of the lower flange (14B) and a flexible steel blad (22B) . This opening permits the front wheels (16) to return to their original position inside the flanges of the u-profile ⁽ 14), just before the stopper ⁽ 21B) arrests the scraper-wagon (15) in its end position. The flexible metal blade (22B) will now support the wheels ⁽ 16 ⁾ of the wagon (15) when it starts a new cycle toward the inlet section. The electric motor (19) is again reversed for a new cycle and the scraper-blade (12) is automatically lowered into its normal scraper position (Posi¬ tion 1) with the support profile (13) in a vertical position.

The traversing scraping and return cycles are continuous, with each cycle taking approximately 1 hour to complete. The return movement speed can be adjusted to be somewhat higher than the scraping speed .

Sludge which is collected in the sludge ^' pocket (10) is pumped periodically out of the tank (1) using a pump (11).

In order to keep the water-surface free from fat, debris and floating scum, a floating scum-scraper ⁽ 24 ⁾ can be connected to the support profile (13) so that the scum-scraper blade ⁽ 241 is..pact ia I ly submerged during the sludge-scraping movement. (Position 1 in Figure 1 ⁾ and is then . raised over the water

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surface level during the return movement ⁽ Position 2 in Figure 1). The collected floating debris or sludge may then be re¬ moved from the tank (1) in various ways such as pumping peri¬ odically out through the tank's service opening ⁽ 25 ⁾ .

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