GB2286824A

GB2286824A - Wastewater treatment

Info

Publication number: GB2286824A
Application number: GB9403832A
Authority: GB
Inventors: Keith Jonathan Ilett
Original assignee: WYATT Ltd
Current assignee: WYATT Ltd
Priority date: 1994-02-28
Filing date: 1994-02-28
Publication date: 1995-08-30
Anticipated expiration: 2014-02-28
Also published as: GB2286824B; GB9403832D0

Abstract

An activated sludge system in which an inorganic adsorbent, preferably powdered activated carbon, is added to aid the removal of substances which are either toxic or inhibitory to the biological process. The adsorbent is introduced (7) after the main oxygenation stage (3). Since peak concentrations of the target substances in the wastewater are reduced by virtue of the mixing and residence time within the oxygenation stage, the quantity of adsorbent needed is advantageously reduced. <IMAGE>

Description

istater Treahknt This invention relates to a process and plant for the treatment of wastewater containing organic materials and, more particularly, for the treatment of industrial wastewaters containing substances which may be either toxic or inhibitory to biological treatment processes.

In wastewater treatment, the chemical oxygen demand (C.O.D.) is frequently used as a measure of the concentration of organic materials and the biological oxygen demand (B.O.D.) is used as a measure of the biodegradable organic materials present, the ratio of C.O.D. to B.O.D. being a measure of the biodegradability of the wastewater and an indicator of the presence, or absence of inhibitory material In the treatment of wastewaters such as those produced in industry, it is well established that the wastewater must be treated to remove not only biologically degradable materials, but also compounds which are either toxic or inhibitory to biological systems.

Various processes and systems have been proposed for treating such wastewaters, including variations of the activated sludge process and, in particular, the addition of inorganic adsorbents to the aeration chaMber of the activated sludge process. (see 'Industrial Wastewater Treatment by the Biophysical Process' BETIEE Conference Toulouse June, 1992).

In the activated sludge process, shown diagramatically in figure 1., wastewater (1) is oxygenated in the presence of biomass in an aeration chamber (3) to allow the biomass to absorb and digest the organic materials in the wastewater. After the oxygenation stage, the biomass and treated wastewater are separated - usually by sedimentation in a settlement tank (4) - and the supernatant liquid (effluent) (2) is discharged from the system. The biomass (return activated sludge) (5) is returned to the inlet of the oxygenation stage for admixture with fresh incoming wastewater.

Two important considerations affecting the efficiency of the process are that a sufficiently high concentration of dissolved oxygen be maintained in the system to prevent the occurrence of septic conditions in the settlement stage and that the activated sludge should be readily settleable to allow for efficient separation from the treated effluent.

The addition of powdered inorganic adsorbent and, in particular, of powdered activated carbon (P.A.C.) to the oxygenation process allows the combination of the adsorbent and the biomass in the activated sludge treatment process to take place in the same reactor.

This combination permits the two processes of physical adsorption by the inorganic adsorbent and biological assimilation by the biomass to work in concert.

The inorganic adsorbent preferentially removes the larger organic compounds such as pesticides which may be toxic or inhibitory to the biological process, these compounds being more slowly degraded by the biomass. Conventional biological activity removes the lower molecular weight compounds which are not readily absorbed by the adsorbent carbon.

The adsorbent also serves as a reservoir for slowly degradable compounds, providing the micro-organisms with more time to assimilate these materials. The net effect is an overall greater removal of toxic and other organic materials present.

The process employing powdered activated carbon is shown in fig. 2 and, as currently practised, it is customary to introduce the activated carbon at any of the three points shown (11), these all resulting in the activated carbon being added prior to, or into, the main oxygenation stage.

Most adsorbents are not specific as to the substances that they adsorb and, in cases where it is required to maintain a very low concentration of a given chemical or chemicals in the treated effluent, the amount of adsorbent required to achieve the required removal may be determined not solely by the concentration of the target substance (s) but also by the concentration of other substances which compete with the target substance(s) for sites on the adsorbent's surface. The amount of adsorbent used must be sufficient to accommodate the concentration peaks of both the target substances and the other substances present.

In the course of the activated sludge process, considerable quantities of competing substances are removed from solution by biological oxidation and adsorption or absorption onto or into the biomass within the activated sludge. Significant quantities of the target substance(s) may also be removed from solution by the same processes, thus reducing both the quantity remaining to be adsorbed and also the quantity of competing substances.

According to the present invention, the adsorbent is added to the activated sludge system at a point between the oxygenation and the solid/liquid separation stages. For example, there may be a separate chamber or chambers (8) provided therebetween as shown in Fig. 3. The said chamber or chambers are mixed - either mechanically or by aeration - to allow for intimate contact between the adsorbent (7) and the liquid phase of the contents and provide sufficient retention time to allow the adsorption process to achieve the required concentration of the target substance (s) in solution. An advantage of adding the adsorbent at a point, for example, to a separate chamber, following the oxygenation stage is that peak concentrations of the target substance(s) in the incoming wastewater will have been reduced by virtue of the mixing and residence time within the oxygenation stage and thereby a less variable concentration of the target substance(s) will require to be absorbed.

Therefore the quantity of adsorbent required will be lessened since it will not be necessary to add adsorbent at concentrations sufficient to satisfy the original peak concentrations.

By monitoring the concentration of the target substance (s) in the soluble form in the waste stream exiting the contacting chamber (8) such that this value can be used to control the dosage of the adsorbent, then such an arrangement would lead to more precise matching of the adsorbent dosage to the concentration of target substance(s) present, leading to economies in the use of adsorbent.

The adsorbent passes to the settlement tank together with the activated sludge and is removed by gravity and returned to the aeration chamber in the conventional manner, where any remaining capacity of the adsorbent may be utilised in removing further substances from solution.

The introduction of the adsorbent with the activated sludge in a separate compartment or compartments, between the oxygenation and solid/liquid separation stage, additionally allows the facility to raise the dissolved oxygen concentration of the contents of this chamber by the injection of air or oxygen, or by other means without the use of excessive power which would be entailed to raise the dissolved oxygen concentration of the whole aeration tank.

This increase in dissolved oxygen level will lead to the possibility of oxygen depletion in the solid/liquid separation stage being substantially reduced and will be particularly beneficial in the event that the adsorbent has a high affinity for adsorbing oxygen from solution at high dissolved oxygen concentrations and desorbing it at low concentrations, as is the case with powdered activated carbon.

With some industrial wastewaters, a difficult to settle biomass (activated sludge) may be produced and it may be necessary to add a material to aid settlement (9), this material being added between the chamber or chambers in which the adsorbent is added and the solid/liquid separation stage.

This material may be added either in a further chamber or in the line between the aeration chamber and the solid/liquid separation stage with the aid of a static or mechanical mixer. This process, in this example using a static mixer (10), is shown diagramatically in fig. 3.

Claims

1. A wastewater treatment process having an oxygenation stage and a solid/liquid separation stage, characterised by the addition of inorganic adsorbent following the oxygenation stage.

2. A wastewater treatment process according to claim 1, wherein the inorganic adsorbent is added to a chamber separate from the oxygenation stage and the solid/liquid separation stage.

3. A wastewater treatment process according to claim 2, wherein the contents of the chamber are mixed mechanically.

4. A wastewater treatment process according to claim 2, wherein the contents of the chamber are mixed by aeration.

5. A wastewater treatment process according to any of claims 2 to 4, comprising monitoring the concentration of the target substance or substances of the inorganic adsorbent in the soluble form in the waste stream exiting the chamber.

6. A wastewater treatment process according to claim 5, further comprising controlling the dosage of the inorganic adsorbent according to the monitored concentration of the target substance or substances.

7. A wastewater treatment process according to any of claims 2 to 6, further comprising the step of raising the dissolved oxygen concentration of the contents of the chamber.

8. A wastewater treatment process according to claim 7, wherein said step consists of the injection of air or oxygen.

9. A wastewater treatment process according to any preceding claim, further comprising the addition, prior to the solid/liquid separation stage, of material to aid settlement.

10. A wastewater treatment process according to any preceding claim, wherein the inorganic adsorbent is powdered activated carbon.

11. Plant for a wastewater treatment process, the plant having an aeration chamber for an oxygenation stage and a tank for a solid/liquid separation stage, characterised by the provision of means for adding inorganic adsorbent following the oxygenation stage.

12. Plant according to claim 11, wherein said means comprises a separate chamber or chambers provided between said aeration chamber and said tank.

13. A wastewater treatment process substantially as hereinbefore described with reference to Figure 3 of the drawings.

14. Plant for a wastewater treatment process, substantially as hereinbefore described with reference to Figure 3 of the drawings.