EP2164809A1

EP2164809A1 - Apparatus and method for precipitation of phosphorus from waste water

Info

Publication number: EP2164809A1
Application number: EP08775534A
Authority: EP
Inventors: Ville Asikainen; Ilkka Kallio; Lauri Palmujoki; Tuomas Pelto-Huikko
Original assignee: Biolan Oy
Current assignee: Biolan Oy
Priority date: 2007-07-05
Filing date: 2008-07-03
Publication date: 2010-03-24
Also published as: CN101730664B; RU2480422C2; CN101730664A; FI20075515A0; RU2010103367A; EP2164809A4; WO2009004123A1; FI119148B

Abstract

This invention relates to an apparatus attachable to a small-scale purification unit for precipitating phosphorus from waste water, said apparatus comprising a pumping vessel (1), a discharge pump (2) for unloading the pumping vessel, a chemical tank (6), a chemical metering pump (5) for dispensing a chemical from the chemical tank (6) into waste water to be discharged from the pumping vessel (1), whereby a separation of the resulting chemical precipitate from water draining out of the purification unit is arranged to be accomplished in a settling tank (12) of the small-scale purification unit. In this invention, the metering pump (5) is arranged to supply a chemical coagulant into an effluent flow discharged by the discharge pump (2) over an entire or almost entire operating cycle of the discharge pump (2).

Description

Apparatus and method for precipitation of phosphorus from waste water

Object of the invention

The object of this invention is an apparatus attachable to a small-scale purification unit for precipitating phosphorus from waste water.

A further object of the invention is a method for precipitating waste water phosphorus with a chemical coagulant from waste water in a small-scale purification unit, said method comprising delivering the waste water to be treated gravitationally into a pumping vessel which is drained by a discharge pump, dispensing the chemical coagulant by a metering pump from a chemical tank into waste water to be discharged from the pumping vessel, whereby a separation of the resulting chemical precipitate from water draining out of the purification unit takes place in a settling tank, or i.e. sedimentation tank, of the small-scale purification unit.

Yet another object of the invention is the use of a foregoing apparatus for precipitating phosphorus from waste water in small-scale purification units, particularly in continuous action small-scale purification units.

Background of the invention

The environmental burden resulting from waste waters in sparsely populated areas has drawn more and more attention over the past few years. Indeed, a decree (542/2003) issued by the Council of State imposes minimum requirements regarding the purification of waste waters. The treatment system existing in most properties does not match the tightened purification requirements. Consequently, the market has experienced the advent of a multitude of various property-specific treatment systems for waste waters, from simple soil treatments all the way to biological-chemical device purification units.

The decree (542/2003) issued by the Council of State, regarding the treatment of household waters in areas isolated from sewer systems, imposes limit values for the amounts of organic matter, total nitrogen, and total phosphorus discharged to the environment. With regard to most treatment methods, it has been discovered in practical studies that the most challenging task in terms of attaining the required purification performance relates to total phosphorus. A reduction of the phosphorus load in sparsely populated areas is significant, since about 8% of the phosphorus load ending up in Finnish water systems originates from scattered permanent and holiday dwellings. Especially in inland waters, phosphorus constitutes a so-called minimum nutrient, regulating the basic production of a lake. The eutrophication caused by a phosphorus load manifests itself in the multiplication of aquatic plants and biomass, as well as in abnormally abundant inflorescences of algae and cyano bacteria, the algae toxins emitted thereby being possible sources of even serious consequences. The decomposition of biomass consumes oxygen, as a result of which the body of water may develop an oxygen depletion leading to fish deaths.

In property-specific effluent treatment systems, the dephosphorization can be enhanced by means of absorption masses and various chemical coagulants. What is commonly used in biological-chemical purification units is an aluminum- or iron-based chemical coagulant. The chemical coagulant binds phosphorus contained in waste water as compounds that can be separated from water by settling. From the standpoint of precipitation efficiency, using a correct amount of chemical is of primary importance. When the dosage is too small, there is not enough chemical to react with all phosphate ions and, thus, some of the phosphorus contained in waste water finds its way into a water system. Excessive overdosage may lower the pH- value of waste water too much as the aluminum- and iron-based chemical coagulants are highly acidic. An excessively low pH-value undermines the efficiency of precipitation. An efficient blending of a chemical coagulant and waste water is a condition for successful precipitation. By virtue of blending, a dose of chemical can be exploited in its entirety, nor is there any need for unnecessary overdosage. In response to a chemical coagulant, the waste water develops microflocs, which may coalesce for increasingly larger floes. Increasing the size of floes is desirable for facilitating the separation thereof from water by settling. This is endeavored by an arrangement, in which the blending is followed by a mixing process. During the course of mixing, the velocity gradient or G-number of water is lower than during the course of blending. The most common way of implementation is to keep the waste water in a slow rotary motion, which in large-scale plants is performed actively by means of slowly rotating blades. The floes present in water must not be exposed to too excessive forces in order to not disintegrate the same as a result of such forces.

Based on the operating principle, the biological-chemical device purification units can be divided into batch- and continuous-action purification units. In a batch purification unit, the steps of a treatment process are carried out in a single tank. It is always the same amount of waste water that is handled by the process, whereby the chemical intended for the precipitation of phosphorus can be dispensed into each batch in a constant amount. This is convenient from the standpoint of pumping systems, since the metering pump shall always have the same running period in a desired process step. In batch purification units, the chemical coagulant is generally dispensed in an aeration step, whereby the turbulence caused by aeration blends the chemical effectively throughout the mass of water.

In continuous-action purification units, the amount of water, which is displaced by water arriving in the system, proceeds gravitationally from one tank to another. In such a system, a technical implementation regarding the dosage of a chemical coagulant is more challenging since the amount of water proceeding from one tank to the next may fluctuate by the instant. There are commercially available products, in which the dosage is organized by means of a timer clock. Controlled by the timer clock, a certain amount of chemical is pumped by a metering pump at desired intervals, for example once an hour. In such a system, the amount of chemical being administered does not depend on the amount of water arriving in the system, but some chemical shall be unnecessarily used, for example during vacation times. The wasteful use of a chemical coagulant incurs extra costs for the property owner. Establishing a dosage, which disregards the amount of water to be processed, may lead to under- or overdose situations. When the amount of water arriving in the system is larger than expected, the amount of administered chemical is not enough for treating the entire amount of water. In addition to extra costs, the overdosage may cause problems resulting from an excessive fall of the pH-value. What is also challenging is the implementation of a sufficiently effective blending process. In commercially available systems, the chemical is usually administered into a conduit with water to be treated flowing therein. In such an arrangement, there is no way of making a difference in the blending conditions of water and chemical. Neither are the conditions optimal for the collision and coalescence of floes.

The Finnish registered utility model No. 7006 discloses a waste water treatment apparatus for the removal of phosphorus, said apparatus comprising a phosphorus treatment unit which includes a process tank, a chemical tank for a phosphorus precipitating chemical, a dispenser for metering the chemical from the chemical tank, a chemical-and-water blending panel provided with flow-controlling partitions, and a blended water-and-chemical collecting conduit extending from a middle section of the blending panel to the process tank below. The water, which is to be cleared of phosphorus in the process tank, is collected according to the example, obviously after a soil treatment, first in a pump well, from which it is then pumped into the process tank for a dephosphorization treatment, followed by conducting the treated water out of the process tank by means of a siphon, not by gravity.

In the solution of FI utility model 7006, a certain amount of chemical is first dispensed into a trough from the chemical tank prior to pumping a certain volume batch of water from the pump well. The water is conducted through the trough, the chemical being picked up by the water as a result of ejector suction and the chemical being carried away from the trough. Downstream of the trough, apparently for ensuring a proper blending process, there is set up a blending panel provided with flow-deflecting partitions.

It is an object of the present invention of ours to provide an apparatus and a method, which, on the one hand, enable the use of a correct and necessary amount of chemical coagulant, thus avoiding the admission of phosphorus, which has failed to react in incomplete precipitation, into a water system, or avoiding the undermining effect of an excessively low pH-value, caused by an excessive amount of chemical, on precipitation efficiency, and which, on the other hand, also enable a sufficiently effective water-and-chemical blending process.

In order to achieve this, an apparatus according to the invention is characterized in that the apparatus comprises a pumping vessel, a discharge pump for unloading the pumping vessel, a chemical tank, a chemical metering pump for dispensing a chemical from the chemical tank into waste water to be discharged from the pumping vessel, whereby a separation of the resulting chemical precipitate from water draining out of the purification unit is adapted to proceed in the small-scale purification unit's settling tank, the metering pump being arranged to supply a chemical coagulant into an effluent flow delivered by the discharge pump over an entire or almost entire operating cycle of the discharge pump. In one preferred embodiment of the invention, the apparatus further comprises a mixing space, the shape of which forces the waste water, containing a chemical coagulant and being passed therethrough, into a slow rotary motion for enhancing the formation of floes produced by the chemical coagulant. Hence, the objective of setting the water-chemical mixture in a slow rotary motion is to enhance the formation of a floe produced by the chemical coagulant prior to delivering the mixture into a precipitation tank, in other words into a settling tank, the purpose of a phosphorus-binding precipitate produced therein being to separate from water and settle down to the tank floor. The shape of the mixing space can be any form capable of setting the chemical-coagulant-containing water in a slow rotary motion that can be either uniformly slow through the entire mixing cycle or become increasingly slower towards the end. Thus, the shape of the mixing space can be any form capable of forcing the chemical-coagulant-containing waste water passing therethrough into a slow rotary motion, such as e.g. a substantially circular form. Examples of a substantially circular form include e.g. a substantially cylindrical i.e. barrel-like form, a substantially spherical or semi-spherical form, a substantially conical or frusto-conical form. Other substantially circular forms include a three-dimensional substantially spiral form. Regardless of the shape, it is the main objective that the chemical- coagulant containing waste water be passed into a slow rotary motion in such a way that at least one wall surface of the mixing space, along which the blending water-chemical mixture is flowing, is substantially circular or curvilinear in terms of its form.

In one particularly preferred embodiment, the mixing space is a substantially cylindrically shaped mixing container. Accordingly, it is also preferred that a waste flow issuing from the pumping vessel, and the amount of chemical coagulant to be supplied therein, be arranged to be delivered to or near the top end of the mixing container, whereby the blending water-chemical mixture flows, while in slow rotation, along the container's circular jacket surface by gravity down towards an outlet opening present in the container bottom.

In another preferred embodiment, the mixing space or container features an outlet opening, which is at a level equal to or higher than the bottom of the mixing container. The mixing space or container can preferably have its outlet opening fitted with a discharge conduit provided with an expansion below the outlet opening. The mixing space or container has its discharge conduit preferably extending below the water level in the settling tank.

In a further preferred embodiment, the discharge conduit features a structure, enabling access of air into the discharge conduit and disabling a development of a continuous water column and suction in the discharge conduit.

The apparatus according to the invention can preferably be positioned downstream of a biological waste water treatment. The biological pretreatment can be a biological process based on the activated sludge method, soil purification, or biological filtration.

The apparatus has its pump system preferably arranged to work in such a way that it includes a float switch for switching the discharge pump on and off according to predetermined maximum and minimum levels of waste water in the pumping vessel, as well as a current control relay coupled between the discharge pump and the metering pump for again switching the metering pump on when the discharge pump is supplied with electric current and off when the discharge pump is not supplied with electric current. Basically, the pumps and the pump system can be constituted by any pumps or pump systems well known for a person skilled in the art. In one preferred embodiment, the apparatus in its entirety is capable of being installed in one and the same settling tank of a small-scale purification unit or, in other words, in a sedimentation tank.

On the other hand, a method according to the invention is characterized in that at least one chemical coagulant is dispensed by the metering pump into an effluent flow delivered by the discharge pump over an entire or almost entire operating cycle of the discharge pump.

In one preferred embodiment for a method of the invention, the waste water, containing a chemical coagulant, is exposed to a mixing process for enhancing the formation of floes produced by the chemical coagulant.

The chemical coagulant can be any to the person skilled in the art known chemical coagulant intended for the removal of phosphorus contained in waste water, e.g. an aluminum- or iron-based chemical coagulant. The chemical coagulant is preferably dispensed in the form of an aqueous solution.

The apparatus and method according to the invention are suitable for use in any small-scale purification units, especially in continuous-action units, comprising at least one sedimentation tank or a number of interconnected sedimentation tanks.

With respect to the available state of the art, the apparatus, constituting the object of claimed protection, provides advantages as follows: -In a continuous-action waste water purification process, the precipitation of phosphorus can be implemented in such a way that a chemical coagulant is only dispensed when the system is supplied with water. -Controlling the operation of a chemical pump on the basis of the operation of a discharge pump enables maintaining the dosage of chemical constant with respect to the amount of water to be treated. -Supplying the water flow to be discharged from the pumping vessel with a uniform dosage of chemical over the entire pumping cycle ensures a consistent chemical concentration as well as a high-grade blending of chemical and water in the water to be delivered into the mixing container. -Precipitation conditions remain unchanged regardless of the amount of water.

-The mixing process in a mixing container connected to the apparatus encourages the growth of floes produced by a chemical coagulant and thereby improves the settling characteristics of resulting sludge. Good settling characteristics facilitate the separation of sludge from treated water and thereby reduce the amount of phosphorus-containing precipitate escaping to the environment along with water being discharged from the system. -Having the water, discharging from the mixing container, to proceed below the water level in a settling tank prevents the migration of precipitation- produced sludge out of the tank along with outgoing water.

The apparatus will now be described more precisely by way of example with reference to the accompanying figure 1, showing one embodiment of the apparatus in a longitudinal section. However, the example presented hereinafter is by no means to be construed as limiting the appended claims in any way.

Detailed description of one embodiment of the invention

Referring to fig. 1, there is shown an apparatus intended for the treatment of waste water, which apparatus can be disposed in a sedimentation tank or a so-called settling tank 12, also referred to as a septic tank, positioned downstream of a continuous-action biological treatment unit intended, for example, for the treatment of waste waters produced by a single property. Water arrives gravitationally in the settling tank 12 by way of a conduit 13 extending thereto, such that the incoming water is first conducted into and collected in a pumping vessel 1 included in the settling tank 12 at its top end. When the water surface rises to a sufficiently high, predetermined maximum level in the pumping vessel 1, a discharge pump 2 included therein has its float switch 3 activate the discharge pump 2 automatically. Pumping is also deactivated by the float switch 3 once the water surface in the pumping vessel 1 has fallen to a desired level i.e. predetermined minimum level. The pumping vessel's 1 capacity is preferably > 20 I. The capacity has an effect on the running frequency of a discharge pump.

The discharge pump's 2 running time is totally dependent on the amount of water arriving in the pumping vessel 1. In the event that water is supplied continuously, such that water in the pumping vessel 1 cannot fall to a desired minimum level, then the discharge 2 shall also be in nonstop operation.

Water is pumped by the discharge pump 2 into a mixing container 4, which is also located inside the settling tank 12. Both the pumping vessel 1 and the mixing container 4 may also be optionally located outside the settling tank 12, but preferably and as presented in this example, such elements constitute an entity which is present inside the settling tank 12 at the top end thereof.

As the discharge pump 2 is running, a desired amount of chemical coagulant is pumped by a metering pump 5 into the mixing container 4 from a chemical tank 6 located inside or outside the settling tank 12. When the discharge pump 2 stops, the metering pump 5 stops as well. The discharge pump 2 and the metering pump 5 are interconnected by means of a current control relay (not shown). By way of the relay, the discharge pump 2 is supplied with electric current all the time. The discharge pump 2 has its float switch controlling the operation of the pump 2, activating and deactivating the pump 2 according to the water level in the pumping vessel 1. The current control relay functions in such a way that, whenever the discharge pump 2 is running, said relay supplies current also to the metering pump 5. Thus, the relay is, so to speak, "sniffing" to discover when the discharge pump takes up current.

The chemical coagulant (aqueous solution) is conducted by way of a tube 14 to the vicinity of a location which marks the termination point of a pipe 7 extending from the discharge pump 2. The pipe 7 terminates at a top end of the mixing container 4 having a substantially cylindrical shape in a horizontal plane, such that the water is guided to make a contact with the mixing container's 4 jacket wall in a substantially parallel relationship therewith. Thus, by virtue of a substantially cylindrical shape of the mixing container 4, the water arriving in the container is forced in to a rotary motion, which the water, for the most part, preferably rolls along a cylindrical vertical wall or in its vicinity down towards the container's bottom, from which it drains or rises to an outlet opening and thereby away. By virtue of this rotary motion, the water to be treated and the chemical coagulant blend effectively with each other. When the discharge pump 2 comes to a halt, the arrival of water in the mixing container 4 comes to a halt, which results in a deceleration of the water rotating speed in the mixing container 4. This promotes the coalescence of floes for larger-size floes. Water runs out of the mixing container 4 through an outlet opening 8 in the middle of the container's bottom, said outlet being flush with or at a level higher than its surroundings, i.e. flush with or higher than the bottom surrounding the outlet opening. The result of this is that the outlet opening 8 in its entirety does not end up below the water level. An air space remaining in the outlet opening 8 prevents the development of suction during the discharge of water. Thus, the dwell time of water in the mixing container 4 is considerably longer than what it would be in the event of the outlet opening 8 finding itself in its entirety below the water level. A longer dwell time encourages the coalescence of floes developed in water in the mixing container 4 in response to a chemical coagulant, which assists in the descent of floes in the settling tank 12. The settling tank's minimum capacity is preferably about 15 I. Finally, it should be noted that another beneficial feature in terms of the coalescence of floes is that the bottom be flat all over or at least over some sort of range around the outlet opening. Thus, the water finds no way of packing itself in the outlet opening.

From the mixing container's 4 outlet opening 8, the water discharges by way of a chamber 9 or an expansion into a discharge pipe 10. The discharge pipe 10 extends to a water delivery pipe 11. The discharge pipe 10 has its end substantially above the water level in the settling tank 12. An air space remaining between the end of the discharge pipe 10 and the water level in the settling tank 12 disables the development of a suction effect which would rapidly evacuate the mixing container 4. The discharge pipe 10 and the delivery pipe 11 assume such relative positions that all of the water coming out of the discharge pipe 10 finds its way into the delivery pipe 11. Hence, the delivery pipe 11 also has its upper end above the water level. An assembly constituted by the discharge pipe 10 and the delivery pipe 11 is referred to as a discharge conduit.

By way of the delivery pipe 11, the water treated with a chemical coagulant discharges to substantially below the water level of the settling tank 12, i.e. substantially below the mouth of a pipe 15 carrying purified water out of the settling tank 12. This is important in a gravitationally working system, because the amount of water going out of the tank matches the amount of water coming in. In order to obtain a good purification result, it is essential that no phosphorus-containing precipitate be snatched along with water going out of the tank. As the water arriving in the tank is delivered to substantially below the water level, some surface water, which is already clarified, shall make an exit from a top section of the tank 12. The heavier- than-water precipitate descends to the tank's bottom. Said lengthening of the water dwell time in the mixing container 4 also reduces the flow rate arriving in the settling tank 12. In this case, naturally, the flow rate out of the tank is also reduced, which decelerates the speed of outgoing water. A low rate of speed is another factor which further reduces a risk of precipitate being snatched along.

Claims

1. An apparatus attachable to a small-scale purification unit for precipitating phosphorus from waste water, characterized in that the apparatus comprises a pumping vessel (1), a discharge pump (2) for unloading the pumping vessel, a chemical tank (6), a chemical metering pump (5) for dispensing a chemical from the chemical tank (6) into waste water to be discharged from the pumping vessel (1), whereby a separation of the resulting chemical precipitate from water draining out of the purification unit is arranged to be accomplished in a settling tank (12) of the small-scale purification unit, the metering pump (5) being arranged to supply a chemical coagulant into an effluent flow discharged by the discharge pump (2) over an entire or almost entire operating cycle of the discharge pump (2).

2. An apparatus as set forth in claim 1, characterized in that the apparatus further comprises a mixing space (4), the shape of which forces the waste water, containing a chemical coagulant and being passed therethrough, in to a slow rotary motion for enhancing the formation of floes produced by the chemical coagulant.

3. An apparatus as set forth in claim 2, characterized in that the mixing space is a substantially cylindrically shaped mixing container (4).

4. An apparatus as set forth in claim 2 or 3, characterized in that the waste flow issuing from the pumping vessel (1), and the amount of chemical coagulant to be supplied therein, are adapted to be delivered to or near the top end of the mixing space or container (4).

5. An apparatus as set forth in any of claims 2 to 4, characterized in that the mixing space or container (4) features an outlet opening (8), which is at a level equal to or higher than the bottom of the mixing space or container (4).

6. An apparatus as set forth in any of claims 2 to 5, characterized in that the mixing space or container (4) has its outlet opening (8) fitted with a discharge conduit (10 and 11) provided with an expansion below the outlet opening (8).

7. An apparatus as set forth in claim 6, characterized in that the mixing space or container (4) has its discharge conduit (10 and 11) extending below the water level in the settling tank (12).

8. An apparatus as set forth in claim 6 or 7, characterized in that the discharge conduit (10 and 11) features a structure, enabling access of air into the discharge conduit and disabling a development of continuous water column and suction in the discharge pipe (10 and 11).

9. An apparatus as set forth in any of claims 1 to 8, characterized in that it is positioned downstream of a biological waste water treatment.

10. An apparatus as set forth in any of claims 1 to 9, characterized in that it includes a float switch (3) for switching the discharge pump (2) on and off, as well as a current control relay coupled between the discharge pump (2) and the metering pump (5) for again switching the metering pump (5) on when the discharge pump is supplied with electric current and off when the discharge pump is not supplied with electric current.

11. An apparatus as set forth in any of the preceding claims, characterized in that the apparatus in its entirety is capable of being installed in one and the same settling tank (12) of a small-scale purification unit.

12. The use of an apparatus as set forth in any of the preceding claims for precipitating phosphorus from waste water in small-scale purification units, particularly in continuous action small-scale purification units.

13. A method for precipitating waste water phosphorus with a chemical coagulant from waste water in a small-scale purification unit, said method comprising delivering waste water to be treated gravitationally into a pumping vessel (1) which is drained by a discharge pump (2), dispensing a chemical coagulant by a metering pump (5) from a chemical tank (6) into waste water to be discharged from the pumping vessel (1), whereby separation of the resulting chemical precipitate from water draining out of the purification unit takes place in a settling tank of the small-scale purification unit, characterized in that at least one chemical coagulant is dispensed by the metering pump (5) into the effluent flow discharged by the discharge pump (2) over an entire or almost entire operating cycle of the discharge pump (2).

14. A method as set forth in claim 13, characterized in that the waste water, containing a chemical coagulant, is exposed to a mixing process for enhancing the formation of floes produced by the chemical coagulant.