Odour Prevention and Control of Organic Sludge and Livestock Farming
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In the vicinity of a rendering plant it is very difficult to find a really unpolluted place for the olfactometric measurements. Although the most unpolluted site luff of the plant was chosen, another precaution was taken. As adaption of the panelists to the plant odour could cause one of the greatest errors, some minutes before and during the measurement the panelists inhale solely odourless air from the olfactometer. To prevent discomfort by inhaling completely dry air, the olfactometer Modell 1158 is supplied with a moistening device, fig. 1. Fig. 1. Moistening device. In a standard impinger, filled with destillated water, air is moistened close to saturation. An equal flow of moistened air is mixed to the olfactometer outlet, thus delivering to the panelist a rel. moisture content of nearly 50 %. The panel consisted of 4 persons. The samples are prediluted taken into plastic bags, simultaneously at the inlet (raw air) and at the outlet (cleaned air) of the air cleaners. To receive an unfalsified sample from the outlet of the biofilters, undiluted by ambient air, a "tent” of plastic foil, fig. 2, is placed on the filter surface. The cleaned air blows up the tent and escapes through the sample hole, large enough to prevent a significant increase of pressure. The form of the upblown tent indicates, wether a sample area with normal air flow is chosen, and over the space of the covered filter area of 6,25m2 an average sample is received. Fig. 2. Device for cleaned air samples from biofilter outlet.
Table II : Quantitative determination of carbonyl compounds at different odour sources (concentrations in ppb) Rendering plant Gelatine plant neighbourhood neighbourhood Formaldehyde 40 16 Acetaldehyde 39 24 Acetone 36 73 Prcpanal 10 -Isobutyraldehyde 10 30 Pentanal 15 19 Hexanal 3.52 Heptanal 12.5 Octanal 10.5 Nonanal 1 2 acids (figure 7). However extractions always involve a serious decrease in sensitivity, while evaporation of the extract produces a solution in 0.1-0.5 ml of solvent, and only 1 pi of it can be brought in the gas chromatograph. Therefore work is in progress to enhance sensitivity by converting acids in to halogenated derivatives, which can be GC-analysed with the more sensitive electron-capture detector. For thiols a similar procedure is investigated as with aldehydes. One possibility is absorption of thiols in an alkaline solution and reaction with 2,4-dinitrochlorobenzene, yielding 2,4-dinitrofenylsulfides, which are analysed by HPLC (9). Sane improvements on removal of reagents at the one hand and on separation of sane by-products on the other hand have to be achieved in order to in crease the sensitivity with another factor of ten. 5. CONCLUSION The actual scope and limitations of chemical analysis of odour show that all problems can be tackled as far as emission is concerned. For iititiission measurements seme progress is necessary, but there is no essential reason why chemical analysis would be unable to attain the desired sensitivity for all types of odorants. There is no doubt that in a few years the last dif ficulties will be solved. In order to achieve real control of odour nui sance, automatic measurement is necessary on a long time basis. There again seme technical development is to be expected. Does this mean that machines are going to decide if an odour is pre sent or not? By no means, while the population will always be the reference, and psychophysical measurements will be necessary to make chemical analysis possible.
be detected specifically, which is possible for sane groups of odorants (thiols or mercaptans, sulphides, amines) with specific GC-detectors. Spe cific detectors are available for haloganted compounds, sulphur-, phosphor-and nitrogen compounds. Figure 4 shews the analysis of the sulphur-ccmpounds produced by the acidic decomposition of phosphate-rock and causing the typi cal smell of fertilizer plants. Another approach is to aim at selective concentration methods. Indeed odour problems are caused by a limited number of compounds, on rather a li mited number of classes of compounds, mentioned in figure 5. For most odour nuisance problems, chemical plants, refineries, live stock production, food processing, rendering, water purification plants etc., the compounds responsible for the odour are known. So chemical analysis of the odour can be limited to these odorants, and selective concentrating techniques can be used. Selective concentrating methods are based on speci fic absorption techniques, using particular chemical reactions of odorant classes. Semet imes several absorption methods have to be used in order to describe the odour problem, thus increasing the labor cost of the analysis. On the other hand absorption methods allow better quantitative results. Se lective absorption of odorants from air produces a far less complex mixture. We developed or are developing several of these methods for aldehydes, amines, acids, thiols etc. Carbonyl ccnpounds for instance can be trapped by absorption in a rea gent solution containing 2,4-dinitrcphenylhydrazine and hydrogen chloride. Details of this method are extensively described elsewhere (8). The prin ciple of the method is that the carbonyl ccnpounds, in case of rendering plant emission the aldehydes, react with the 2,4-dinitrophenylhydrazine and form 2,4-dinitrophenylhydrazones (2,4-DNPH's) according to the scheme. These 2,4-dinitrophenylhydrazones have seme interesting properties. It are cristalline caipounds so that after extract of the 2,4-DNPH's fran the reagens, they can be concentrated by evaporation of the solvent without losing product. Besides these caipounds shown intense absorption of UV-light (X 356 nm) and so they can easily be detected with an UV-detec-tor. These properties make the 2,4-DNPH's particularly suitable for HPDC-analyse. This methods is used since seme time. A chranatogram is given in figure 6 and results of the quantitative determination of carbonyl com pounds in different situations are given in table 2. For amines absorption in an acid solution, or preferably adsorption onto an acid ion exchange column (acidified divinylbenzene-styrenesulfo-nic acid copolymer) is used. 10-50 1 of ambient air is sent over*a wet 100nnix3irmI.D. column; the ion exchange polymer is put into a vial, made alkaline and the water solution is analysed on packed Carbowax-KDH GC-column with a thermionic selective detector (TSD), which is specific for nitrogen- and phosphorus-catpounds. Trimethylamine is detected easi ly at 1 ppb. Aibids can be absorbed specifically in an alkaline impringer, which is extracted with ether after acidification to pH 2. This method was used for rendering plant emissions, shewing a series of linear and branched
through tubing and fittings made of PTFE. Analysis was undertaken by the Warren Spring Laboratory of the Department of Trade and Industry, according to the method described by Bailey and Bedbo rough The results are shown in Table IV. and plotted in Fig. 3. and 4. Table IV. Variation of odour strength of extracted samples with volune of eluted air Volume of air Strength of odour samples passing through (dilutions) sludge before sampling (1/1) Raw sludge Digested sludge 0 154 000 9 900 11.1 53 000 350 22.2 30 600 270 55.6 15 500 190 111 8 200 160 It is clear from these results that there is considerable die-off of odour strength with time, and that, as would be expected, the anaerobic digestion of sludge can reduce the odour potential by at least one order of magnitude. To illustrate the importance of this die-off effect, the results have been re-plotted in Fig. 5. in a cunulative form; that is to say as cumulative percentage of the eventual colour release against volume of air. In the case of the raw sewage sludge, 38% of the ultimate odour was carried in the first odour sample, and 90% of the odour had been extracted by the passage of about 200 1. In the case of the anaerobically digested sludge, the same effect is much more marked; 72% of the ultimate odour was carried by the first sample, and thereafter the strength of the odour fell off very rapidly. There are two possible explanations for this. First, it can be postulated that as it is known that many of the important odorous chemical species are highly volatile, they may be only physically trapped in the sludge, and need little encouragement to transfer to the atmosphere. An alternative explanation concerns the existence of two equilibria. As the vapour/liquid equilibrium is disturbed by the passage of air, the concentration of dissolved compounds in the liquid phase falls, disturbing the ’solid’/liquid equilibrium The kinetics of transfer across this latter phase boundary are much slower than for the liquid/vapour transfer, so that the extraction of odour becomes limited by the rate of diffusion into the liquid phase. Two observations may be cited as evidence for this latter view. First, when sludge is applied to land, there is a rapid tail-off of odour nuisance after spreading. Hie incidence of rain after a dry period is known to result in an increased evolution of odour. Second, in earlier experiments samples of sludge were centrifuged, and the supernatant liquor discarded and replaced by tap water, before being used in the standard odour potential test. Some re-extraction of odour from the samples was rapidly found. In practice, both postulated mechanisms are probably at work, especially if the concept of ’solid/liquid equilibrium’ be extended to
