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An evaluation of methods for assessing the removal efficiency of a grit separation device
~
Pergamon
War. Sci. Ttclt. Vol. 33. No.9. pp. 269-275. 1996. Copynght @ 1996 IA WQ. Published by ElseVIer SCIence Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + 000
PH: S0273-1223(96)00396-4
AN EVALUATION OF METHODS FOR ASSESSING THE REMOVAL EFFICIENCY OF A GRIT SEPARATION DEVICE Peter Gardner and Andrew Deamer Hydro Research and Development Ltd.• Clevedon BS2J 7RD. UK
ABSTRACT The paper considers the many problems which can beset any attempt to verify or measure the performance of a grit removal device. including contractual ambiguities. the difficulties of assessing any flow dependency. sampling difficulties. uncertainty about analytical techniques and efficiency definitions. The different definitions of efficiency are discussed and it is concluded that the 'partial penetration' would be the most applicable to the situation. The usefulness is discussed of attempting to separate grits which do not damage machinery and improve the handleability ofthe primary sludge. In the measurement of efficiency only two process streams need be sampled. Different protocols are considered which specify which streams should be sampled. and the advantages and disadvantages of each are set oul. Solid stream sampling of recovered grit and primary sludges over a lengthy period is rejected as an unwieldy technique which may be impracticable on certain sites. The analysis of sludges for a small quantity of grit is problematic and subject to error, but this is also true of settled liquid stream samples. Rather it is concluded that supplementary grit add.tion coupled with liquid effluent stream sampling would give an efficiency subject to little error. The results of recent significant work carried out in the testing of a Grit King R Separator, both in-situ and under controlled conditions. using different methods are presented in this paper. The recommended technique is described together with the sampling methods and further work required to verify/validate the procedure. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Background grit; efficiency; grading; grit; measurement; sampling; separation; standard analysis method; supplementary grit addition.
INTRODUCTION In most urbanised areas. the sewage system is such that the underground pipes carry both foul sewage, derived from domestic sources. and storm runoff, from streets and paved areas. Consequently the sewage arriving at the treatment works comprises not only organic material but grit and sand, especially during storm conditions when the debris on the surface is re-eroded. A limited amount of fine grit has been shown to have a beneficial effect on the 'handleability' of primary sewage sludge. but where there is an excessive quantity of grit or where the sewage treatment process incorporates pumps and other machinery, there is a corresponding need to remove the large grit particles from the sewage at an early stage in the treatment to avoid damage and wear to that machinery (Metcalf and Eddy. 1991). 269
270
P. GARDNER and A. DEAMER
PROBLEMS WITH DEFINITIONS The design of wastewater treatment plants has tended towards modularity in recent years, with process modules being sourced from different suppliers and connected such that, together, they form the complete treatment process. Thus contracts are drawn up so that purchaser and supplier both know and understand exactly of what the equipment in question should be capable. This emphasis on contractual specification has, in some cases, obscured the design requirement that sufficient grit be removed to avoid damage and excessive wear to pumps and machinery. For example, the removal of 'grits' down to 75~1.m may be seen by some not only as unnecessary (such particles do not damage pumps) but also counter-productive (the sludge handleability is diminished). Performance specifications then, tend to define not only the specific gravity (density) and size of the grit particles which the device should remove, but also the efficiency with which that removal should be effected. Bearing in mind that the objective should be to prevent damage and wear, it might be argued that the 'efficiency' should rather be expressed as an 'inefficiency' ie that no more than a specified mass or proportion of grit should be able to progress past the grit removal stage of the treatment process. Logic notwithstanding, the efficiency could be expressed as one of a number of different definitions; total, partial, grade or reduced (Svarovsky, 1990). These are defined as the fraction of the mass of the grit in the influent liquor which is retained by the device, the fraction of that grit in the influent liquor which is between two defined sizes (e.g. 150~m and 300~m) and which is retained by the device, the fraction of that grit of a single size in the influent liquor which is retained by the device, and the efficiency of the device after the linear scale 0 ~ 100% has been mapped to the linear scale flow split ~ 100%. The different definitions can sometimes lead to confusion if the performance specification is loosely worded: "95% efficiency of all particles down to l50~m" can mean either that the partial efficiency (5.0mm to 150~m) or the grade efficiency (i.e. for single size) for all sizes from 5.0mm to 150~m should be 95%. Misunderstandings are possible even if the specification is more carefully worded: "95% efficient at a particle size of 150~m" cannot in practice be measured directly but must be approximated by the partial efficiency either between 212~m and 150~m or between 150~m and 105~m (212~m and 105~m being the nearest available sieve sizes). PROBLEMS WITH TESTING Once the client and supplier have agreed the performance to be achieved, there are many difficulties with verifying the achievement of the required performance. The situation is simplified if the testing can be done before it is 'wet' commissioned (i.e. before sewage is run through it), but even then there are non-trivial logistical problems to be dealt with. In the case when the testing must be done after such commissioning, there are more problems. Often the client's specification calls for tests to be performed at the design flow rate but makes no allowance as to how this should be achieved. In some cases the site operators can be persuaded to 'play tunes with the pumps' to obtain full flow for a few minutes, but this short duration precludes some methods of testing. If there are no pumps feeding the inlet works then the grit extraction device will be subject to the usual diurnal and seasonal variations in flow rate and grit load. Sewage works are not often designed with the needs of scientific sampling in mind, and it is usually difficult (when the flow rate is large) either to sample the entire flow for a short period of time, which might otherwise be accomplished at a free outfall, or to obtain representative sub-samples. In the latter case, stratification of the flow will result in the inorganic material moving along the bottom of a channel (British Standards Institution, 1980) and the organic material towards the top, where it is difficult to sample, even using a rectangular bucket. Some methods require knowledge of the mass and grading of grit present in the sewage as it enters the grit removal device, and this 'background level' is usually determined prior to when the tests proper are
Assessing removal efficiency of grit separator
271
performed. Consequently it must be assumed that the quantity and quality of grit remains consistent throughout the test, which may not be valid for tests of long duration. Once samples have been acquired they must be analysed. Whilst there are well researched and documented methods for the analysis of suspended solids, moisture etc (American Public Health Association, 1989), there does not appear to be such for the analysis of a sample of raw or settled sewage for the mass and grading of inorganic particles in it. Whilst this may, on first inspection, seem quite trivial, the fact that organic matter does not disappear on ashing (at 550°C) but merely reduces to particles smaller in size, and as such is indistinguishable from inorganic grit, means that some means of separation of organic from inorganic particulates before ashing is usually required. This is where sceptics will notice that one principle of separation is being used in the measurement of the efficiency of another. AVAILABLE METHODS A typical grit removal and primary sedimentation process is shown schematically as figure I. The grit removal installation is shown as incorporating a grit classifier (secondary separator), the liquid effluent of which is returned to the influent (liquid) stream, and the solid effluent being the grit recovered (solid stream). The sludge and sewage effluents from the primary sedimentation device are described as primary sludge and primary effluent respectively. PRIMARY ...------- EFFLUENT .-----------'---~
INFLUENT - - , - - - - 1
PRIMARY 5EDIMENT ~ ~
GRIT REMOVAL
1 L-..
GRIT [LA,WIER • \/
------r
---~1
PRIMARY SLUDGE
-'
[LA5'.IFIER DECANTRA lE
RECOVERED
GRIT
Figure I. Schematic layout of grit removal and primary treatment processes.
The various methods of determining the grit removal efficiency of the grit removal device involve sampling one or more of the different process streams and may involve supplementary grit addition to the influent stream. The methods were reviewed by Bedwell, 1993 and are outlined as follows: Solid/solid stream samplin~ This technique involves monitoring of the recovered grit and the primary sludge stream. It is assumed that all grit which is not caught by the grit removal device is caught by the primary sedimentation device, and that, consequently, the two devices between them catch all of the grit in the influent stream. LiQujdlsolid (underflow) samplinl: Supplementary grit addition is coupled with collection and analysis of the recovered grit to enable the grit recovery efficiency to be determined. Extensive monitoring of the background grit can sometimes replace the supplementary grit addition. This method incorporates those which sample either the liquid effluent from or the liquid influent to the grit removal device, since the liquid stream sampling is only used to assess the level of background grit.
P. GARDNER and A. DEAMER
272
LiQuid/licluid (overflow)
samplin~
This testing method utilises supplementary grit addition to the influent and sampling (pumped or bucket) of the effluent stream from the grit removal device. Sampling is also carried out on the influent stream of the grit removal device to assess the levels of background grit entering the process. DISCUSSION OF THE METHODS Solid/solid stream samplinl: This method is preferred and recommended by some workers, but in practice is a very lengthy and unwieldy method. Long term (the long retention times of the primary sedimentation process require this) accumulation of grit in the primary sludge is compared with that recovered by the grit separation device to obtain a removal efficiency for the grit removal device. The method does not allow for the flow dependence of the grit removal device to be investigated since a constant flow rate cannot be maintained for such long periods (Le. weeks). If there is more than one primary sedimentation device fed by the grit removal device under study then they must all be monitored. There are problems with the analysis of the primary sludge, as discussed previously, and the sampling of a sludge stream in a representative manner carries with it its own special problems since it is likely that grit will not be evenly distributed in the sludge. The method requires no supplementary grit addition and does not require separate tests to determine the 'background' grit level since the 'background' grit becomes the test material for this method. In this respect it could be argued that the test is more realistic, since it uses real sewage grit as the test material, but the consequence of this is that there is very little grit at the upper and lower ends of the size spectrum and hence reduced accuracy of the partial efficiency measurements for those size bands. The principle advocates of the method give details of a method of analysis of primary sludge for grit (Wilson and Chang, 1985), but the critical analysis of the method is beyond the scope of this paper. LiQuid/solid (underflow)
samplin~
This method will tend to be of medium to long duration due to the retention time of grit in the grit dewatering unit (e.g. grit may take several days to travel up the barrel of a screw classifier). Invariably some grit is 'lost' in the system. either in the pipework or inside the grit removal device. or new grit merely displaces old grit (for example in the grit which forms the 'bed' in a classifier barrel) such that what comes out is not what was put in. Some knowledge is required of the quantities of grit in the sewage, either by direct determination of the background level or by estimation e.g. from historical records of the number of grit skips removed over the previous few months. In the latter case it must be ensured that the quantity of grit already in the sewage is 'small' (i.e. at least two orders of magnitude smaller than) compared with the quantity added. Ligujdl1jgujd (overflow)
samplin~
The requirements about the knowledge of the level of background grit are as those for liquid/solid stream test methods (Le. it must be measured or estimated). The method is of short duration, and is therefore suitable for use at sites where the required flow rate can only be maintained for a short period of time (e.g. 5 minutes). In this respect the assumption about the background grit level (constant during the test proper) is more likely to be valid. The flow dependency of the grit removal system can be investigated if the flow rate can be controlled. The sub-sampling of a liquid stream such that the grit in the sub-sample is representative of that in the process stream is very difficult, and it is more desirable repeatedly to sample for a very short period of time the whole process stream. Recent work, however, indicates that the use of a simple, hand• operated pump can be used, with correction for the stratification of the flow, to obtain representative sub• samples where the flow rates are so high as to preclude the sampling of the whole stream. The analysis of the samples is non-standard but does not involve the analysis of sludge. The realism of this method might be
Assessing removal efficiency or grit separalor
273
said to lie in the direct measurement of that which the grit removal device is installed to prevent, i.e. the grit which is passed on to downstream treatment processes. Although the performance specification might be expressed as an efficiency of grit capture, the measure of the inefficiency (referred to as the 'penetration') in this way may well find favour in the future as being nearer to the problem which it the grit removal device is installed to cure. The relative advantages and disadvantages of the three methods can be summarised as Table I. Table I. Relative merits of different sampling protocols solid/solid
liquid/solid
liquid/liquid
weeks
days
minutes
different flows possible?
X
X
t/
knowledge of background level necessary?
X
.....
.....
supplementary grit addition required?
X
.....
t/
very hard
hard
hard
duration
sample analysis COMPARISON
As a part of its normal research activities, Hydro Research and Development has undertaken tests on a number of installations of its Grit King R Separator (Andoh, 1993) grit removal device. In the interests of evaluating different methods of measurement of the grit removal effiCiency, a number of methods were used during these tests. A comparison of the methods used at the two sites is subsequently described. Grit King R Separator performance between sites cannot be directly compared because they are designed to meet the client's specific requirements. which have varied from site to site (e.g. "95% removal down to 150 m grit of specific gravity 2.65" at one site and" ..down to 300 m.. Of specific gravity 2.5" at another).
The Grit KingR Separator was tested in situ using the liquidlliquid stream sampling protocol, and samples were taken using both a bucket and a hand operated pump, for which the intake velocity had been matched to the velocity of the bed load. Table 2 shows a summary of the results of the testing. Table 2. Comparison of bucket and pumped sampling Size range
Partial efficiency for range
Partial efficiency down to smaller size in range Bucket Pump Bucket Pump 100% 100% 100% >300j.tm 100% ISD-3ool'm 95% 91 % 99% 98% I05-150j.tm 87% 78% 98% 97% 75-1051-tm 80% 73% 97% 96% < 75J.!m 78% 80% 97% 96% The results demonstrate that the two different sampling methods yield very similar partial efficiencies, especially when the measure in question is the partial efficiency down to a certain particle size.
274
P. GARDNER and A. DEAMER
Testing on this Grit King R Separator used clean water and proceeded under controlled conditions at a temporary testing facility. This enabled the liquid/solid stream sampling protocol to be compared with the liquidlliquid stream protocol. Tests were performed at two flow rates (the higher flow being more than 5 times higher than the lower flow rate). and the results are summarised as Tables 3 and 4. Table 3. Comparison of sampling protocols: lower flow Lower flow
Partial efficiency for range
Partial efficiency down to smaller size in range size band under over under over > 595J'm 156.3% 100.0% 156.3% 100.0% 420-595J'm 99.6% 100.0% 101.8% 100.0% 297-420Jlm 84.7% 99.8% 86.8% 99.8% 21O-297Jlm 107.6% 99.8% 97.8% 99.8% 114.3% 99.7% 99.9% 99.8% I77-210ILm 149-177Jlm 95.8% 99.7% 99.4% 99.8% 105-149Jlm 107.6% 99.1 % 99.9% 99.7% 105-74Jlm 95.1 % 94.3% 99.9% 99.7% < 74/Lm 62.5% 56.3% 99.9% 99.6% Table 4. Comparison of sampling protocols: higher flow
Higher flow
Partial efficiency for range
Partial efficiency down to smaller size in range size band under over under over > 595Jlm 90.0% 100.0% 90.0% 100.0% 420-595Jlm 85.3% 100.0% 85.5% 100.0% 297-420Jlm 88.6% 95.0% 88.1 % 95.8% 210-297Jlm 78.1% 79.2% 82.4% 86.5% 177-21OJ'm 55.5% 58.3% 78.5% 82.3% l49-l77JLm 44.9% 45.0% 74.5% 77.9% 105-149J'm 31.3% 17.4% 71.5% 73.8% 105-74JLm 21.2% -17.6% 71.0% 72.8% < 74JLm 28.6% -107.1 % 71.0% 72.7% The comparison of the efficiencies measured using the two methods is not, at first glance, very impressive. It is believed that the partial efficiencies measured for the specific size bands using the different methods may have been are different due to the agglomeration of some grit particles. thereby artificially reducing and increasing respectively the partial efficiencies for two. adjacent size bands. It should also be noted that the efficiency of the Grit King R Separator at the lower flow rate is very close to 100% which may have caused error. In general. errors are greater when the efficiency is close either to 0% or to 100%. Once again the partial efficiencies down to a certain size compare very well. The mass balances for grit were very good: of the 441 kg of grit added in each case, only 0.2% and 1.7% respectively was unaccounted for. This either indicates that both methods are equally accurate, or that both methods are equally inaccurate! The former is taken to be true. CONCLUSION AND RECOMMENDATIONS Given the objective for the removal of grit at a sewage treatment works (to protect downstream machinery). the removal of silts and sands (Le. grit which is smaller than 100 Jlm is not only unhelpful but is actually a hindrance. The various problems have been highlighted associated with ambiguities in the definition of
Assessing removal efficiency of grit separator
215
removal efficiency and the interpretation of these definitions in order to be able to perform in situ verification of design performance. The most suitable method for the measurement of the grit removal efficiency of a grit removal device has been shown to be supplementary grit addition to the influent stream coupled with sampling of the liquid effluent stream (liquid/liquid stream sampling protocol). The entire liquid effluent stream should be sampled for short periods of time wherever possible. or. where this is not possible. using a hand operated pump with the inlet velocity matched to the bed velocity. The accuracy of the method has been estimated to be 2%. The need has been highlighted for further work in the field, including defining a standard method for the analysis of settled sewage and sewage sludges for grit content. the stratification of open channel flow and the need to weight the sub-sampling using a pump according to the expected grit loads at various heights above the channel bed. Also relevant is the continuing need to define the problem: whilst the need to remove grit at a sewage works is self-evident. there is little information on the particle size distribution of typical sewage grit.
REFERENCES American Public Health Association (1989). Standard methods for the examination of water and wastewater (the 'Blue Book'). 17th edition Andoh RYG (1993) Hydrodynamic grit separation· Grit King R Separator. Hydro Research and Development. Bedwell. C. E. (1993). A Study of Methods for Assessing the Grit Removal Efficiency of a Vonex Separator. MSc Thesis. Imperial College. London. Brillsh Standards Institution (1980). Methods of measurement of liquid flow in open channels: Bed material sampling. BS 3680: Part IOC: 1980. Metcalf and Eddy (ed Tchobanoglous) (1991). Wastewater Engineering' Treatment, Disposal, Re-use. McGraw-Hili. Svarovsky. L. (1990). Solid-liquid separation. 3rd edition. Butlerworths. Wilson. G. E. and Chang, D. P. (1985). In·Situ Measurement of the Flow Dependence ofMuniciple Abrasives (Grit) Loads. Eutek Systems. June.
M1')):W
Pergamon
War. Sci. Ttclt. Vol. 33. No.9. pp. 269-275. 1996. Copynght @ 1996 IA WQ. Published by ElseVIer SCIence Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + 000
PH: S0273-1223(96)00396-4
AN EVALUATION OF METHODS FOR ASSESSING THE REMOVAL EFFICIENCY OF A GRIT SEPARATION DEVICE Peter Gardner and Andrew Deamer Hydro Research and Development Ltd.• Clevedon BS2J 7RD. UK
ABSTRACT The paper considers the many problems which can beset any attempt to verify or measure the performance of a grit removal device. including contractual ambiguities. the difficulties of assessing any flow dependency. sampling difficulties. uncertainty about analytical techniques and efficiency definitions. The different definitions of efficiency are discussed and it is concluded that the 'partial penetration' would be the most applicable to the situation. The usefulness is discussed of attempting to separate grits which do not damage machinery and improve the handleability ofthe primary sludge. In the measurement of efficiency only two process streams need be sampled. Different protocols are considered which specify which streams should be sampled. and the advantages and disadvantages of each are set oul. Solid stream sampling of recovered grit and primary sludges over a lengthy period is rejected as an unwieldy technique which may be impracticable on certain sites. The analysis of sludges for a small quantity of grit is problematic and subject to error, but this is also true of settled liquid stream samples. Rather it is concluded that supplementary grit add.tion coupled with liquid effluent stream sampling would give an efficiency subject to little error. The results of recent significant work carried out in the testing of a Grit King R Separator, both in-situ and under controlled conditions. using different methods are presented in this paper. The recommended technique is described together with the sampling methods and further work required to verify/validate the procedure. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Background grit; efficiency; grading; grit; measurement; sampling; separation; standard analysis method; supplementary grit addition.
INTRODUCTION In most urbanised areas. the sewage system is such that the underground pipes carry both foul sewage, derived from domestic sources. and storm runoff, from streets and paved areas. Consequently the sewage arriving at the treatment works comprises not only organic material but grit and sand, especially during storm conditions when the debris on the surface is re-eroded. A limited amount of fine grit has been shown to have a beneficial effect on the 'handleability' of primary sewage sludge. but where there is an excessive quantity of grit or where the sewage treatment process incorporates pumps and other machinery, there is a corresponding need to remove the large grit particles from the sewage at an early stage in the treatment to avoid damage and wear to that machinery (Metcalf and Eddy. 1991). 269
270
P. GARDNER and A. DEAMER
PROBLEMS WITH DEFINITIONS The design of wastewater treatment plants has tended towards modularity in recent years, with process modules being sourced from different suppliers and connected such that, together, they form the complete treatment process. Thus contracts are drawn up so that purchaser and supplier both know and understand exactly of what the equipment in question should be capable. This emphasis on contractual specification has, in some cases, obscured the design requirement that sufficient grit be removed to avoid damage and excessive wear to pumps and machinery. For example, the removal of 'grits' down to 75~1.m may be seen by some not only as unnecessary (such particles do not damage pumps) but also counter-productive (the sludge handleability is diminished). Performance specifications then, tend to define not only the specific gravity (density) and size of the grit particles which the device should remove, but also the efficiency with which that removal should be effected. Bearing in mind that the objective should be to prevent damage and wear, it might be argued that the 'efficiency' should rather be expressed as an 'inefficiency' ie that no more than a specified mass or proportion of grit should be able to progress past the grit removal stage of the treatment process. Logic notwithstanding, the efficiency could be expressed as one of a number of different definitions; total, partial, grade or reduced (Svarovsky, 1990). These are defined as the fraction of the mass of the grit in the influent liquor which is retained by the device, the fraction of that grit in the influent liquor which is between two defined sizes (e.g. 150~m and 300~m) and which is retained by the device, the fraction of that grit of a single size in the influent liquor which is retained by the device, and the efficiency of the device after the linear scale 0 ~ 100% has been mapped to the linear scale flow split ~ 100%. The different definitions can sometimes lead to confusion if the performance specification is loosely worded: "95% efficiency of all particles down to l50~m" can mean either that the partial efficiency (5.0mm to 150~m) or the grade efficiency (i.e. for single size) for all sizes from 5.0mm to 150~m should be 95%. Misunderstandings are possible even if the specification is more carefully worded: "95% efficient at a particle size of 150~m" cannot in practice be measured directly but must be approximated by the partial efficiency either between 212~m and 150~m or between 150~m and 105~m (212~m and 105~m being the nearest available sieve sizes). PROBLEMS WITH TESTING Once the client and supplier have agreed the performance to be achieved, there are many difficulties with verifying the achievement of the required performance. The situation is simplified if the testing can be done before it is 'wet' commissioned (i.e. before sewage is run through it), but even then there are non-trivial logistical problems to be dealt with. In the case when the testing must be done after such commissioning, there are more problems. Often the client's specification calls for tests to be performed at the design flow rate but makes no allowance as to how this should be achieved. In some cases the site operators can be persuaded to 'play tunes with the pumps' to obtain full flow for a few minutes, but this short duration precludes some methods of testing. If there are no pumps feeding the inlet works then the grit extraction device will be subject to the usual diurnal and seasonal variations in flow rate and grit load. Sewage works are not often designed with the needs of scientific sampling in mind, and it is usually difficult (when the flow rate is large) either to sample the entire flow for a short period of time, which might otherwise be accomplished at a free outfall, or to obtain representative sub-samples. In the latter case, stratification of the flow will result in the inorganic material moving along the bottom of a channel (British Standards Institution, 1980) and the organic material towards the top, where it is difficult to sample, even using a rectangular bucket. Some methods require knowledge of the mass and grading of grit present in the sewage as it enters the grit removal device, and this 'background level' is usually determined prior to when the tests proper are
Assessing removal efficiency of grit separator
271
performed. Consequently it must be assumed that the quantity and quality of grit remains consistent throughout the test, which may not be valid for tests of long duration. Once samples have been acquired they must be analysed. Whilst there are well researched and documented methods for the analysis of suspended solids, moisture etc (American Public Health Association, 1989), there does not appear to be such for the analysis of a sample of raw or settled sewage for the mass and grading of inorganic particles in it. Whilst this may, on first inspection, seem quite trivial, the fact that organic matter does not disappear on ashing (at 550°C) but merely reduces to particles smaller in size, and as such is indistinguishable from inorganic grit, means that some means of separation of organic from inorganic particulates before ashing is usually required. This is where sceptics will notice that one principle of separation is being used in the measurement of the efficiency of another. AVAILABLE METHODS A typical grit removal and primary sedimentation process is shown schematically as figure I. The grit removal installation is shown as incorporating a grit classifier (secondary separator), the liquid effluent of which is returned to the influent (liquid) stream, and the solid effluent being the grit recovered (solid stream). The sludge and sewage effluents from the primary sedimentation device are described as primary sludge and primary effluent respectively. PRIMARY ...------- EFFLUENT .-----------'---~
INFLUENT - - , - - - - 1
PRIMARY 5EDIMENT ~ ~
GRIT REMOVAL
1 L-..
GRIT [LA,WIER • \/
------r
---~1
PRIMARY SLUDGE
-'
[LA5'.IFIER DECANTRA lE
RECOVERED
GRIT
Figure I. Schematic layout of grit removal and primary treatment processes.
The various methods of determining the grit removal efficiency of the grit removal device involve sampling one or more of the different process streams and may involve supplementary grit addition to the influent stream. The methods were reviewed by Bedwell, 1993 and are outlined as follows: Solid/solid stream samplin~ This technique involves monitoring of the recovered grit and the primary sludge stream. It is assumed that all grit which is not caught by the grit removal device is caught by the primary sedimentation device, and that, consequently, the two devices between them catch all of the grit in the influent stream. LiQujdlsolid (underflow) samplinl: Supplementary grit addition is coupled with collection and analysis of the recovered grit to enable the grit recovery efficiency to be determined. Extensive monitoring of the background grit can sometimes replace the supplementary grit addition. This method incorporates those which sample either the liquid effluent from or the liquid influent to the grit removal device, since the liquid stream sampling is only used to assess the level of background grit.
P. GARDNER and A. DEAMER
272
LiQuid/licluid (overflow)
samplin~
This testing method utilises supplementary grit addition to the influent and sampling (pumped or bucket) of the effluent stream from the grit removal device. Sampling is also carried out on the influent stream of the grit removal device to assess the levels of background grit entering the process. DISCUSSION OF THE METHODS Solid/solid stream samplinl: This method is preferred and recommended by some workers, but in practice is a very lengthy and unwieldy method. Long term (the long retention times of the primary sedimentation process require this) accumulation of grit in the primary sludge is compared with that recovered by the grit separation device to obtain a removal efficiency for the grit removal device. The method does not allow for the flow dependence of the grit removal device to be investigated since a constant flow rate cannot be maintained for such long periods (Le. weeks). If there is more than one primary sedimentation device fed by the grit removal device under study then they must all be monitored. There are problems with the analysis of the primary sludge, as discussed previously, and the sampling of a sludge stream in a representative manner carries with it its own special problems since it is likely that grit will not be evenly distributed in the sludge. The method requires no supplementary grit addition and does not require separate tests to determine the 'background' grit level since the 'background' grit becomes the test material for this method. In this respect it could be argued that the test is more realistic, since it uses real sewage grit as the test material, but the consequence of this is that there is very little grit at the upper and lower ends of the size spectrum and hence reduced accuracy of the partial efficiency measurements for those size bands. The principle advocates of the method give details of a method of analysis of primary sludge for grit (Wilson and Chang, 1985), but the critical analysis of the method is beyond the scope of this paper. LiQuid/solid (underflow)
samplin~
This method will tend to be of medium to long duration due to the retention time of grit in the grit dewatering unit (e.g. grit may take several days to travel up the barrel of a screw classifier). Invariably some grit is 'lost' in the system. either in the pipework or inside the grit removal device. or new grit merely displaces old grit (for example in the grit which forms the 'bed' in a classifier barrel) such that what comes out is not what was put in. Some knowledge is required of the quantities of grit in the sewage, either by direct determination of the background level or by estimation e.g. from historical records of the number of grit skips removed over the previous few months. In the latter case it must be ensured that the quantity of grit already in the sewage is 'small' (i.e. at least two orders of magnitude smaller than) compared with the quantity added. Ligujdl1jgujd (overflow)
samplin~
The requirements about the knowledge of the level of background grit are as those for liquid/solid stream test methods (Le. it must be measured or estimated). The method is of short duration, and is therefore suitable for use at sites where the required flow rate can only be maintained for a short period of time (e.g. 5 minutes). In this respect the assumption about the background grit level (constant during the test proper) is more likely to be valid. The flow dependency of the grit removal system can be investigated if the flow rate can be controlled. The sub-sampling of a liquid stream such that the grit in the sub-sample is representative of that in the process stream is very difficult, and it is more desirable repeatedly to sample for a very short period of time the whole process stream. Recent work, however, indicates that the use of a simple, hand• operated pump can be used, with correction for the stratification of the flow, to obtain representative sub• samples where the flow rates are so high as to preclude the sampling of the whole stream. The analysis of the samples is non-standard but does not involve the analysis of sludge. The realism of this method might be
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said to lie in the direct measurement of that which the grit removal device is installed to prevent, i.e. the grit which is passed on to downstream treatment processes. Although the performance specification might be expressed as an efficiency of grit capture, the measure of the inefficiency (referred to as the 'penetration') in this way may well find favour in the future as being nearer to the problem which it the grit removal device is installed to cure. The relative advantages and disadvantages of the three methods can be summarised as Table I. Table I. Relative merits of different sampling protocols solid/solid
liquid/solid
liquid/liquid
weeks
days
minutes
different flows possible?
X
X
t/
knowledge of background level necessary?
X
.....
.....
supplementary grit addition required?
X
.....
t/
very hard
hard
hard
duration
sample analysis COMPARISON
As a part of its normal research activities, Hydro Research and Development has undertaken tests on a number of installations of its Grit King R Separator (Andoh, 1993) grit removal device. In the interests of evaluating different methods of measurement of the grit removal effiCiency, a number of methods were used during these tests. A comparison of the methods used at the two sites is subsequently described. Grit King R Separator performance between sites cannot be directly compared because they are designed to meet the client's specific requirements. which have varied from site to site (e.g. "95% removal down to 150 m grit of specific gravity 2.65" at one site and" ..down to 300 m.. Of specific gravity 2.5" at another).
The Grit KingR Separator was tested in situ using the liquidlliquid stream sampling protocol, and samples were taken using both a bucket and a hand operated pump, for which the intake velocity had been matched to the velocity of the bed load. Table 2 shows a summary of the results of the testing. Table 2. Comparison of bucket and pumped sampling Size range
Partial efficiency for range
Partial efficiency down to smaller size in range Bucket Pump Bucket Pump 100% 100% 100% >300j.tm 100% ISD-3ool'm 95% 91 % 99% 98% I05-150j.tm 87% 78% 98% 97% 75-1051-tm 80% 73% 97% 96% < 75J.!m 78% 80% 97% 96% The results demonstrate that the two different sampling methods yield very similar partial efficiencies, especially when the measure in question is the partial efficiency down to a certain particle size.
274
P. GARDNER and A. DEAMER
Testing on this Grit King R Separator used clean water and proceeded under controlled conditions at a temporary testing facility. This enabled the liquid/solid stream sampling protocol to be compared with the liquidlliquid stream protocol. Tests were performed at two flow rates (the higher flow being more than 5 times higher than the lower flow rate). and the results are summarised as Tables 3 and 4. Table 3. Comparison of sampling protocols: lower flow Lower flow
Partial efficiency for range
Partial efficiency down to smaller size in range size band under over under over > 595J'm 156.3% 100.0% 156.3% 100.0% 420-595J'm 99.6% 100.0% 101.8% 100.0% 297-420Jlm 84.7% 99.8% 86.8% 99.8% 21O-297Jlm 107.6% 99.8% 97.8% 99.8% 114.3% 99.7% 99.9% 99.8% I77-210ILm 149-177Jlm 95.8% 99.7% 99.4% 99.8% 105-149Jlm 107.6% 99.1 % 99.9% 99.7% 105-74Jlm 95.1 % 94.3% 99.9% 99.7% < 74/Lm 62.5% 56.3% 99.9% 99.6% Table 4. Comparison of sampling protocols: higher flow
Higher flow
Partial efficiency for range
Partial efficiency down to smaller size in range size band under over under over > 595Jlm 90.0% 100.0% 90.0% 100.0% 420-595Jlm 85.3% 100.0% 85.5% 100.0% 297-420Jlm 88.6% 95.0% 88.1 % 95.8% 210-297Jlm 78.1% 79.2% 82.4% 86.5% 177-21OJ'm 55.5% 58.3% 78.5% 82.3% l49-l77JLm 44.9% 45.0% 74.5% 77.9% 105-149J'm 31.3% 17.4% 71.5% 73.8% 105-74JLm 21.2% -17.6% 71.0% 72.8% < 74JLm 28.6% -107.1 % 71.0% 72.7% The comparison of the efficiencies measured using the two methods is not, at first glance, very impressive. It is believed that the partial efficiencies measured for the specific size bands using the different methods may have been are different due to the agglomeration of some grit particles. thereby artificially reducing and increasing respectively the partial efficiencies for two. adjacent size bands. It should also be noted that the efficiency of the Grit King R Separator at the lower flow rate is very close to 100% which may have caused error. In general. errors are greater when the efficiency is close either to 0% or to 100%. Once again the partial efficiencies down to a certain size compare very well. The mass balances for grit were very good: of the 441 kg of grit added in each case, only 0.2% and 1.7% respectively was unaccounted for. This either indicates that both methods are equally accurate, or that both methods are equally inaccurate! The former is taken to be true. CONCLUSION AND RECOMMENDATIONS Given the objective for the removal of grit at a sewage treatment works (to protect downstream machinery). the removal of silts and sands (Le. grit which is smaller than 100 Jlm is not only unhelpful but is actually a hindrance. The various problems have been highlighted associated with ambiguities in the definition of
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removal efficiency and the interpretation of these definitions in order to be able to perform in situ verification of design performance. The most suitable method for the measurement of the grit removal efficiency of a grit removal device has been shown to be supplementary grit addition to the influent stream coupled with sampling of the liquid effluent stream (liquid/liquid stream sampling protocol). The entire liquid effluent stream should be sampled for short periods of time wherever possible. or. where this is not possible. using a hand operated pump with the inlet velocity matched to the bed velocity. The accuracy of the method has been estimated to be 2%. The need has been highlighted for further work in the field, including defining a standard method for the analysis of settled sewage and sewage sludges for grit content. the stratification of open channel flow and the need to weight the sub-sampling using a pump according to the expected grit loads at various heights above the channel bed. Also relevant is the continuing need to define the problem: whilst the need to remove grit at a sewage works is self-evident. there is little information on the particle size distribution of typical sewage grit.
REFERENCES American Public Health Association (1989). Standard methods for the examination of water and wastewater (the 'Blue Book'). 17th edition Andoh RYG (1993) Hydrodynamic grit separation· Grit King R Separator. Hydro Research and Development. Bedwell. C. E. (1993). A Study of Methods for Assessing the Grit Removal Efficiency of a Vonex Separator. MSc Thesis. Imperial College. London. Brillsh Standards Institution (1980). Methods of measurement of liquid flow in open channels: Bed material sampling. BS 3680: Part IOC: 1980. Metcalf and Eddy (ed Tchobanoglous) (1991). Wastewater Engineering' Treatment, Disposal, Re-use. McGraw-Hili. Svarovsky. L. (1990). Solid-liquid separation. 3rd edition. Butlerworths. Wilson. G. E. and Chang, D. P. (1985). In·Situ Measurement of the Flow Dependence ofMuniciple Abrasives (Grit) Loads. Eutek Systems. June.
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