AU701363B2 - Removal of nutrients from effluents - Google Patents

Description

-2- This invention relates to a process for the removal of nutrients from effluents, especially those resulting from food processing. The invention is particularly concerned with the removal of nutrients, especially nitrogen and phosphorus.

The disposal of sludge from activated sludge processes by conventional means such as dewatering by centrifugation or belt filtration (as shown in Diagram 1 of the accompanying drawing) poses economic problems for small scale operations. The complex equipment commonly used for disposal of sludge from domestic sewage treatment plants becomes excessively expensive on a small scale due to capital, maintenance and operating costs of the relatively specialised equipment.

In contrast to domestic sewage, abattoir effluent contains a high ratio of carbon to nitrogen and phosphorus making it feasible and simple to remove approximately 90% ofN P by the aerobic activated sludge process the nutrients being Sincorporated into the sludge or biomass in the course of conventional bacterial growth (see Diagram However, due to the large amount of carbonaceous matter present in the effluent, excessive amounts of sludge are so produced and must be suitably treated before disposal. It is difficult and costly to concentrate the sludge so produced by centrifugation or filtration on a small scale, due to the complex equipment required.

Stabilising this sludge for disposal by conventional anaerobic digestion is also costly on a small scale and the sludge remaining after digestion must still be concentrated 25 before ultimate disposal. Anaerobic digestion also leads to the liberation of N P from oooo• S"the digested cells, thus reforming a large part of the soluble N P originally present in the effluent. On the other hand, while treatment of abattoir effluent in anaerobic ponds, prior to treatment by conventional aerobic activated sludge or aerobic lagoons, is effective in reducing the amount of sludge produced in the previously mentioned aerobic treatments, such, anaerobic treatment reduces the ratio of carbon to nitrogen and phosphorus entering the previously mentioned aerobic treatments, leaving the majority of the nutrients in solution after such treatment. Thus in neither case is the objective of 941 130,p:\oper\tgc,tw5 l62.cap,2 p:\OpER\MJC\79153-94,SPE 9112198 -3- N P removal achieved.

The present invention seeks to overcome the above problems in a simple manner which is cost effective on a small scale. This has been achieved using a combination of known methods for the removal of nitrogen and phosphorus compounds from effluent with disposal of the sludge so produced to an upstream anaerobic environment, the effluent from which is fed back to the conventional nutrient removal process.

According to one aspect of the present invention, there is provided a process for removal of compounds of nitrogen and compounds of phosphorus from an effluent having a BOD/TKN ratio greater than or equal to 3 which comprises the steps of: subjecting the effluent to an aerobic/anoxic bacterial treatment, such as an activated sludge treatment, in a manner known to remove N P, to produce a treated liquor for discharge and a sludge or biomass; transferring the sludge or biomass so produced to an anaerobic environment allowing the sludge or biomass to settle while being stabilised by .0 15 anaerobic action; and returning the supernatant liquid from step to step Step may be achieved, conveniently, by passing the sludge into a covered 0.9* anaerobic lagoon. Preferably the sludge or biomass is allowed to settle in the anaerobic environment for a period of at least 10 days, more preferably at least 30 days. This period 20 is referred to herein as the "sludge age".

As used herein the term "anaerobic environment" is to be understood to refer to an environment which is substantially free of dissolved oxygen and which contains a population of anaerobic bacteria sufficient to stabilise the sludge or biomass by anaerobic action.

A typical process in accordance with the invention is shown in Diagram 2. The addition of lime to the influent, or addition of aluminium or iron salts to the activated sludge process can be employed, where required, to provide for phosphorus removal.

It is well known that only a minor proportion of nutrients are removed when domestic sewage is treated in conventional aerobic processes. This is due to the ratio of nitrogen and phosphorus to carbon in domestic sewage being much higher than that in the bacterial biomass produced. More recently, it has been found that manipulation of the conditions used in conventional aerobic activated sludge processes can lead to removal of nitrogenous -compounds as nitrogen gas, in addition to that removed with the biomass -4- (Cooper et a. 1977, Chong and Giles 1979). However, this method still requires a certain minimum ratio of carbon to nitrogen to be present in the influent. Using biochemical oxygen demand (BOD) as a measure of carbon compounds and Kjeldahl Nitrogen (TKN) as a measure of nitrogen compounds, the minimum theoretical ratio of carbon to nitrogen in the influent to such processes, expressed as BOD/TKN, is about 3. This ratio compares to the ratio of about 10 found in abattoir effluent for example.

In the treatment of food processing effluents, such as abattoir effluent, it is desirable to optimise the ratio of BOD/TKN in the influent to the above described nutrient removal process in order to minimise the amount of sludge produced therein.

This objective may be achieved, in accordance with another embodiment of the present invention, by subjecting all or part of the effluent to an anaerobic treatment before passing it to the activated sludge treatment in step above. Conveniently, a conventional anaerobic lagoon may be used for this purpose, as shown in Diagram 3.

Alternatively, the preliminary anaerobic treatment can be carried out in the anaerobic lagoon used for disposal of sludge from the nutrient removal process as shown in Diagram 4, thereby to obtain the simultaneous reduction of part of the carbonaceous material present in the effluent in order to optimise the BOD/TKN ratio and hence minimise the amount of sludge produced in the nutrient removal process. A concomitant benefit will be a reduction in the amount of oxygen transfer needed. However, in the latter two cases, the anaerobic treatments must be operated in such a manner that the minimum BOD/TKN ratio of about 3 is achieved in the influent to the nutrient removal (aerobic) process in order to achieve efficient removal of compounds of nitrogen.

S"By selecting the appropriate ratio of effluent which is passed directly to the aerobic treatment to that which is subjected to preliminary anaerobic treatment, it is possible to minimise the nitrogen discharge and power consumption in the aeration step.

The invention is further described in and illustrated by the following non limiting examples.

941130,p:\oper\tgctw5162.cap,4 Example 1 In a treatment system corresponding to Diagram 4, abattoir effluent containing levels of Chemical Oxygen Demand (COD) and Total Kjeldahl Nitrogen (TKN) ranging from 1,400 to 3,000 mg/L and from 190 to 360 mg/L respectively was fed each week day to an unstirred anaerobic digester running at an hydraulic residence time of 10 days. The effluent from this digester was fed to a cyclic activated sludge system every week day during the non aerated period of 12 hours. The system was aerated overnight for 12 hours, settled for 1 hour and the clear supemrnatant treated liquor was decanted off. The conditions of operation used in the cyclically aerated activated sludge system were an hydraulic residence time of 5 days and a sludge age of 30 days. These values were chosen from conventional activated sludge nitrification design theory (Barnes D. and Bliss P.J. "Biological control of Nitrogen in Wastewater Treatment" E. F.N. Spon London 1983). A calculated volume of mixed liquor was withdrawn each week day from the activated sludge system to maintain the sludge age at 30 days. This mixed liquor was recycled to the anaerobic digester described above.

The effluent from the activated sludge unit contained greatly reduced levels of compounds of nitrogen, typically 1 to 20 mg/L of ammonia and 40 to 90 mg/L of nitrate as N.

ooo o Example 2 a In a modified system similar to that of Example 1, the operation of the activated 25 sludge section was changed by feeding equal volumes of abattoir effluent and anaerobic S"digester effluent to it, the total volume and all other operating conditions remaining the same. The hydraulic residence time in the anaerobic digester was maintained at 10 days by discarding excess effluent to waste.

The effluent from the activated sludge unit contained even lower levels of compounds of nitrogen to those achieved by the process of Example 1; typically 1 to mg/L of ammonia and 10 to 40 mg/L of nitrate as N.

941130,p:\oper\tgc,tw5162.cap,5 -6- Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

References: Nitrogen removal from wastewater. Cooper, Drew, Bailey, D.A. and Thomas, E.V. (1977) Recent advances in sewage effluent denitrification. Part 1. Water Pollution Control 76, pp 389-401 Chong, R.W.T. and Giles, J.H. 1979. Development of the B4,000 A Single :.**.Vessel Sequentially Operated Extended Aeration Wastewater Treatment Plant. Proceedings of the Eighth Federal Convention, Australian Water and Wastewater Association, Gold Coast, Queensland.

*o *o o* 941130,p:\oper\tgc,tw5162.cap,6 P:\OPERMJC\79153-94.SPE 7/112/98 -7- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A process for removal of compounds of nitrogen and compounds of phosphorus from an effluent having a BOD/TKN ratio greater than 3 which comprises the steps of: subjecting the effluent to an aerobic/anoxic bacterial treatment in a manner known to remove N P, to produce a treated liquor for discharge and a sludge or biomass; transferring the sludge or biomass so produced to an anaerobic environment and allowing the sludge or biomass to settle while being stabilised by anaerobic action; and returning the supernatant liquid from step to step S' 2. A process as claimed in Claim 1, wherein the bacterial treatment in step is an activated sludge treatment.

e ee 15 3. A process as claimed in Claim 1 or Claim 2, wherein step is effected by transferring the sludge into a covered anaerobic lagoon.

A process as claimed in any one of the preceding Claims, wherein lime and/or aluminium and/or iron salts are added to the effluent prior to or during step to effect phosphorus removal.

A process as claimed in any one of the preceding Claims, wherein all or part of the effluent is subjected to a preliminary anaerobic treatment before step 6. A process as claimed in Claim 5, wherein the preliminary anaerobic treatment is carried by feeding the effluent into an anaerobic lagoon.

7. A process as claimed in Claim 5, wherein the same anaerobic lagoon is also used to effect step

Claims (2)

1. 8. A process according to any one of claims 1 to 7 wherein the effluent is a food processing effluent.
2. 9. A process according to any one of claims 1 to 8 wherein the sludge or biomass is allowed to settle in the anaerobic environment for a period of at least 10 days. 9. A process for removal of compounds of nitrogen and compounds of phosphorus from a food processing effluent substantially as hereinbefore described with reference to the Examples and/or Drawings 2, 3 or 4. a DATED this 9th day of December, 1998 Commonwealth Scientific and Industrial Research Organisation By DAVIES COLLISON CAVE Patent Attorneys for the Applicant ABSTRACT A process for removal of compounds of nitrogen and compounds of phosphorus from a food processing effluent which comprises the steps of: subjecting the effluent to an aerobic/anoxic bacterial treatment in a manner known to remove N P, to produce a treated liquor for discharge and a sludge or biomass; passing the sludge or biomass so produced to an anaerobic environment and allowing the sludge or biomass to settle while being stabilised by anaerobic action; and returning the supernatant liquid from step to step 9. 9* «o *o oD 941130,p:\oper\tgc,tw5162.cap,9