IE49424B1 - Treatment of sulphurous sewage with hydrogen peroxide - Google Patents

Abstract

Process for desulphuration of sewage containing sulphides and hydrogen sulphide, which process comprises introducing hydrogen peroxide at several stations in a network of sewers which supply the sewage to a treatment plant, the said stations being regularly distributed over the entire network of sewers.

Description

This invention relates to a process for treating sewage containing sulphides and/or hydrogen sulphide.

The presence, in sewage, of sulphide products which are generally formed by the reduction of sul5 phates under the effect of anaerobic bacteria has a twofold disadvantage: the release of nauseating odours of a toxic product, hydrogen sulphide, and considerable corrosion of purifying equipment and of pipes conveying the sewage, whether made of metal or concrete.

Various methods of treatment have long been proposed for eliminating sulphide products from sewage, such as aeration and oxygenation.

‘ The use of hydrogen peroxide HgOg as an oxidising reagent for treating sewage was advocated in U.S. Patent No. 2,070,856; the oxidation of H2S and sulphides by means of HgOg is in itself a well-known reaction.

The -treatment of sulphurous sewage with hydrogen peroxide has been proposed in French Patent No. 2 ,126,199 which advocates a treatment comprising introducing H202 at a single point in the sewage system, or at the entrance to a treatment plant, which comprises, successively, a preliminary conditioning phase in which 4 to 10 times the stoichiometric equivalent of the sulphide and hydrogen sulphide in the sewage taken in is used, then a period of cleaning treatment in which 1 to 2 times this stoichiometric equivalent are used.

In certain cases, using this process, it is possible to obtain, at the entrances of the treatment plants sewage containing virtually no sulphides or hydrogen sulphide, but the process does not ensure satisfactory protection of the entire network of sewers upstream of the purifying station.

We have found that, to obtain good results in general, both with regard to the protection of the sewage network and with regard to reducing the content of sulphides or hydrogen sulphide at the entrances to the purifying plants, it is necessary to introduce the hydrogen peroxide at seve'ral stations in the network of sewers supplyingthe sewage to the treatment plant, the said stations being regularly distributed over the entire network of sewers.

The quantities of required cannot be laid down a priori, as they depend not only on the sulphide content of the sewage to be treated but also on the characteristics of the particular network of sewers, for example, the length of the drains, the average retention time and the temperature. 9 4 2 4 Our.tests on sewage networks during operation have shown that the best use of ^2^2 ls a°bisved by distributing its introduction over various stations so that the sulphide- content of the sewage at the entrance to the various placeswhere the H202 is added is practically zero.

The stations at which H202 is introduced should preferably be selected so that treatment begins as far upstream as possible in the network of sewers and covers the entire network of sewers as uniformly as possible .

The retrieval or collection stations of the network are stations which are particularly suitable for the injection of the H202 as the operation of the H202 injection pumps can then be synchronised with that of the retrieval pumps, and the agitation created .by the retrieval pumps is sufficient to ensure satisfactory homogenisation of the mixture.

Any aqueous solution of H202 can be used to per20 form the process according to the invention, but it is particularly recommended to use 50% H202s°lutions, which result in the the lowest costs.

The introduction of H202 into the sewage drains can be carried out by any suitable method; the use of metering pumps is particularly recommended.

The following Examples are given by way of further illustration of the process according to the invention. Example 1 is a comparative example in which the H202 is introduced at a single point in the network .

Example 2 illustrates the practical use of the process of the invention for treating sulphide-containing sewage with H202, with the hydrogen peroxide being introduced at several stations of the network of sewers supplying the sewage, the stations being distributed regularly over the entire network.

EXAMPLE 1 (Comparative) A network of sewers supplying a biological purification plant consists of three retrieval or collection stations B, C and D receiving the sewage from the various districts of a town; the sewage from these three points is combined at a central retrieval or collection station A by means of the following drains: (i) BA (gravity drain) 1650 m long and 0.315 m in diameter (ii) CA comprising two parts: a pressurised duct 665 m long and 0.25 m in diameter, then a gravity section 905 m long and 0.25 m in diameter (iii) DA comprising two parts: a pressurised duct 900 m long and 0.355 m in diameter, then a gravity section 1,690 m long and 0.355 m in diameter.

The central collection station A is itself connected to a purification plant via a pressurised duct 1,870 m long and 0.40 m in diameter. The purification plant receives other sewage, hut the checks made at its entrance were made only on the sewage coming from collection station A.

During the test period, the sewage coming from 5 station A has a flow rate of 3,000 to 4,000 m^/day, and its characteristics, in the absence of any oxygenating treatment, are as follows: Temperature pH Redox potential Sulphide content on entering collection station A: to 22°C 7.4 to 8.4 -60 to -400 mV 0.52 mg/1 on average Sulphide content on entering treatment plant: 4.6 mg/1 on average The measurements of sul-phide are measured with a potentiometer immediately after the samples are taken, using a sulphide-selective electrode.

The treatment with H202, performed by introducing hydrogen peroxide via metering pumps into the central collection station A, was carried out over 42 days, by introducing 50% aqueous H202 solution after the re25 trieval pumps. The H202 pumps are synchronised with the sewage retrieval pumps, which operate intermittently.

This intermittent operation results in an average retention time of 100 minutes in the outlet for the period in question. Samples of the sewage were taken from the tank upstream of the retrieval pumps and. level with the flow from the outlet at the entrance to the plant.

The operating conditions and the results obtained in certain periods during this test are shown in Table I below.

After 2 days’ operation without any addition of H202, a sulphide level at the entrance to collection station A varying from 0.32 to 0.61 mg/1 is observed, and a level varying from 4.2 to 5.2 mg/1 is measured at the entrance to the purification plant; on the third day, for a preliminary conditioning treatment, 22.9 mg/1 of 50% H202, i.e. about 2.5 times the stoichiometric amount of sulphur for the oxidation reaction of the sulphides, are added at A. On the 4th day , the H202 content has fallen to 5.7 mg/1, then on the 7th day it has risen to about' 8.8 mg/1 of H202· As the sulphide level at the entrance to the plant is irregular and too high, on the 9th day the H202 content is increased to about 19 mg/1, and this content is maintained until the 18th day.

During the test, an average consumption of 50% H202 of 15.1 mg per m of sewage entering the plant is noted, whilst the sulphide content at the entrance to the plant varies considerably.

The test was continued for over a month with variable H202 contents, and on the 42nd day of operation 494 24 it is found that, with an content of 36.1 mg/1 of 50% Hg'Og, the sulphide content has been reduced to less than 0.1 mg/1, as shown in Table I.

This result, which is satisfactory as regards the sulphide content at the entrance to the treatment plant, is obtained with a quantity of ^Og corresponding to 3-9 times the stoichiometry of the oxidation reaction, but on the other hand this process does nothing to eliminate odours and corrosion in the pipes upstream of collection station A.

EXAMPLE 2 A sewer network is composed, as in Example 1, of a system of drains meeting at collection stations B, C, D which are connected together at a collection station A, and further includes another system meeting at _a station E. The drains leaving A and Ξ are connected to each other at the entrance to the treatment plant.

The characteristics of sections BA, CA, DA and of station A are as indicated in Example 1, and station E is in turn connected to the drain coming from A just before the entrance to the treatment plant, by means of a pressurised duct 850 m long and 0.25m in diameter.

Durjng the test period, the sewage arrives at the purification plant at a total flow rate of 6,700 to 7i3OO m^/day, of which 5,500 to 6,000 m^/day come from station A, and its characteristics in the absence of any oxygenating treatment are as follows: Temperature 20°5 C to 23°C PH 7.55 to 8.50 Redox potential - 260 to - 400 mV Sulphide content at entrance to station A 0.35 mg/1 on average Sulphide content at entrance to stations B, C, D, Ξ less than 0.05 mg/1 Sulphide content at entrance to treatment plant · 3.43 mg/l on average The sulphides are measured using a potentiometer immediately after the samples are taken, by means of a sulphide-selective electrode.

A test on the treatment of the sewage with HgOg, in which the peroxide was introduced into the five collection stations A, B, C, D and E simultaneously, was carried out over 9 days, by introducing a 50% aqueous HgOg solution after the retrieval pumps, using metering pumps. The H202 pumps are synchronised with the sewage retrieval pumps, which operate intermittently.

The operating conditions and results obtained are shown in Table II below.

The H202 is initially introduced at station A, then progressively at the other stations. The sulphide content at the entrance to station A is initially 0.35 mg/l, then becomes less than 0.05 mg/1 in two days, and in parallel fashion the sulphide content at the entrance to 48424 the purification plant, which is 3.4 mg/1 on average just before the Injection of H202 is started, becomes less than 0.05 mg/1 when H202 has been injected into the five stations A, B, C, D and E.

It will be noted that the use of the present process leads to sulphide contents of virtually zero, below the limit for their analytical detection, namely 0.05 mg/1 over the entire network of sewers. This treatment thus results in the complete disappearance of odours and eliminates all corrosion throughout the entire system of sewers.

During this test, the average consumption of 50% χ H202 was 36.5 g of 50% aqueous H202 per m of waste entering the treatment plant, and this means that the sulphide content at the entrance to this same plant can be reduced from 3-4 mg/1 to less than 0.05 mg/1.

TABLE I Example I: Introduction of H202 at one point of the network of sewers.

Day Time Collection Station A Sulphide level at entrance oi purification plant (mg/l) Sulphide level at entrance (mg/l) Level of treatment (mg/l of 50% H202) 1 11h20 4.4 l4h40 4.2 15h10 0.51 l6h10 0.61 2 10h20 0.32 · I4h25 5.2 '5 9h5O 0.70 22.9 10h25 - 22.9 1.9 13h5O 22.9 0.6 l6h05 22.9 2.6 4 9h55 0.57 5-7 12h00 5.7 1.7 l4h50 5.7 3.05 7 13h50 8.8 8 9h00 8.8 5.7 10h45 8.8 2.86 111x30 0.33 8.8 12h00 8.8 4.16 15h15 8.8 3.64 I6h35 8.2 4.94 9 5h30 8.2 7.28 7h30 8.2 6.5 9h3O 8.2 1.8 10h15 0.57 8.2 11h15 8.2 3.1 15h55 19.2 2.55 18h00 19.2 3.28 21h45 19.2 ' 0.26 10 6h10 19.2 6.24 11h10 19.2 3.04 l6h10 19.2 1.48 21h30 19.2 40.1 11 8h45 19.2 5-7 10h30 19.2 0.26 TABLE I (continued) Example I: Introduction of H202 at one point of the network of sewers.

Day Time Collection Station A Sulphide level at entrance of purification plant (mg/1) Sulphide level at entrance (mg/l) Level of treatment (mg/1 of 50% h2o2) 13 12h00 0 (break- · down) 14 11h00 19.2 4.42 l4h30 19.2 2.2 17h00 19.2 2.88 15 8h30 19.2 3.67 l4h00 • 19.2 1.71 16 7h00 19.9 6.03 12h00 19.9 2.62 22h00 19.9 0.73 17 7h00 19.9 4.28 10h30 19.9 1.43 21h30 19.9 0.30 18 7h00 19.9 3.95 12h00 19.9 0.98 38 11h25 16.6 2'. 88 15h3O 16.6 2.88 17h35 16.6 3-67 39 8h50 16.6 4.45 12h30 16.6 0.80 I4h35 16.6 0.62 17h45 16.6 0.39 18h00 36.1 42 8h50 36.1 z.0.1 11h35 36.1 $0.1 15h00 36.1 <0.1 18h00 36.1 $0.1 Example 2: Introduction of Η-,Ο- at various points of the ω +5 ω ι c Ό (0 Ο -Η O •H fi fi-H v CH® 3+J-Pr a +i ο ti h 3 Φ fi O Station E O rH to ε rH ω > φ rH -px-s fi CM 0) o S CM +) sc CB Eh in mmm o’ o o Station D OO OO OOO v v roo oo ooo mmm v r- v Station C in in CM CJ oo Station B m mmm v- r- v r ' Station A rH -P faO α ε ca ω in cm o fi di Ή CM H H o W in in m mm mco com mmo otnin cn cncn cnco coo ooko kO\-v m mm mr- v- cm cmcmcm cm Sulphide level at entrance (mg/1) -! Time ί oooo oo oo oo ooo oom momo mm mm mm mmm nor ifirp 33 3! 35 35 Λ 35 35 35 35 35 35 cri S’ a v- oj m TABLE II (conti nued) Example 2 .- Introduction of HgO- at various points of the network of w ω Φ W CD P Φ 1 G Ό π Ο Ρ O •I- G G --i m m in in kO in in mm cm m m X? r- CO 3 Ρ Ρ H o o OO o oo oo o o ft Q h ft co c\ > Ρ ϋ co ho o o oo o o o o o 6 ό o 3 ω CO H c V V x/v w vv V V G c •r- tflv-r co co co mmm mm mmm m P o into • to H in in in mmmm mm mmm m P co d dd mm d>d-dd- c in in m mmmm mm c r- r- r- * · » • · ♦ · • · mmm m •I—1 mmm OOO oooo oo kOkOO kO P V- V“ in in in in in in in mm to Q B P O CO bO C ε o in in in in in r~ r- r- s- ISISS IS Ή • · · • » V r- 5Γ· · · • P CM CM CM CM CM CM CM CMkOkO kOkO kOkOkO kO 03 P ISC-IS s-is CM CM CM CM CM CM CM CM P Φ co u > P • P^ G CM c ω o co CO CO t— γ— V T“ V v- m co co co CO o B t\i • ·<—} •P x mmm mmko kOkOkD CM cm m t£> to to kO <0 r- s- m mmcM CM CM cm m mcM v v- to m tuTS. P hO co B in P P bo G S Φ*·—' in in in in . mm m e in in in in CM CM cm cm cm m mm mmm m Ρ H O'-' • to φ m cm S— V- r- in m mmmm mm in co co co ω > o ί-, ω M ej • r- T- v- V T“ V V“ V S— v- 7“ Β Ρ Ο X g o Φ P ID •H •d to o P H G^ in in in in m in mm in co iri tOH o o o o o o o o o P Φ • · • · • • • · • co r Η > P fed o o Ο o o o oo o 3 Φ G ε CO H Q)'-' M V V v V V V V OOO OOO mmmo o m Ο Ο ΙΪΊ o Φ ootn OOIC r- v~ v m m OOt- o 6 rrr rrr rrrr ff ΧίΧίΛ r-kO S χί P od-to r- cod- tri to o in cn B V V V T- r- r~ r~ V T~ V V“ >, CO cn O χ— CM in kO Q T~ V“ Cont'd 48424 TABLE II (continued.) Example 2 : Introduction of Ho0o at various points of the network of sewers ω ρ φ ι £ Ό rt Ο ·Η O •H C L Ή X3H CO ft 0) Sh Pi CO G η > Ρ ϋ rt hO φ ‘H -H H £ « Η φ Ο Ή ft-' mm mm o o o o o d do V\J vv Station E mm mm mm mm CJ o •H P CO +5 WQ O kOkO kOkO e-o >-i> Station C rd \ hO ε H Φ > Φ H kOkO kOkO CM CM CM CM cj O ♦H P CO P CO CQ P'-* C CM ΦΟ ε cm P ffi co ω £> k O E-ι in CO cr\ CFiCh 'CC co co v* v- V- T- rH P hO α ε O'-' g s& Ρ H O'co φ in cm Φ > o jl, φ ch cm ε-iH Offi m in m in in in in in ai co co co <Γ- V V“ V ' Station A Sulphide level at entrance' (mg/1) m m o o o o V v Time oo om o m o rΛΛ ΛΛ mo v- Day ko ts v- v

Claims (4)

1. CLAIMS 1 . Process for the desulphuration of sewage containing sulphides and hydrogen sulphide, which process comprises introducing hydrogen peroxide at several stations in a network of sewers which convey sewage to a treatment plant, the said stations being regularly distributed over the entire network of sewers,
2. 2. A process according to Claim 1, wherein the distribution of the hydrogen peroxide between the various 10 stations is such that the sulphide and hydrogen sulphide contents of the sewage at the entrance to the various stations where H 2 0 2 is introduced are practically zero.
3. 3. A process according to any one of Claims 1 and 2, wherein 50% by weight hydrogen peroxide is used. 15
4. 4. A process for the desulphurisation of sewage containing sulphides and hydrogen sulphide, substantially as hereinbefore described in Example 2.