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Settleable Solids Test

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odors  will  develop.  If  this  occurs,  you  should check  for  flow  problems  within  the  sanitary system  upstream  of  the  treatment  plant. HYDROGEN  ION  CONCENTRATION (pH  VALUE)  TEST The measure of acidity or basicity (alkalinity) of  something  is  called  the  pH.  The  effect  of  pH on some parts of wastewater treatment makes it an   important   test.   A   low   pH   of   domestic wastewater  may  mean  that  the  wastewater  is septic, or it can mean that industrial or commer- cial acid wastes are entering the system. A pH of 6.5 to 8 is about right for treatment plant influent. Test results showing a very high or low pH may mean someone is breaking sewer use regulations. Sudden  changes  of  0.5  or  more  on  the  pH  scale may mean that operating problems are starting. Grab  samples  should  be  taken  for  pH  tests. SETTLEABLE SOLIDS TEST The settleable solids test on wastewater can tell the operator a lot about what kind of wastewater is  coming  into  the  plant  and  how  the  solids  are settling. Also, the settleable solids test can help the operator estimate the volume of sludge to be expected  in  the  clarifier. Either  grab  or  composite  samples  will  work for  this  test.  The  test  is  done  using  an  Imhoff cone.  The  Imhoff  cone  (fig.  10-1)  can  be  either glass or plastic. It can hold 1 liter and is marked off  in  milliliters  (ml). Before running the test, you should allow the sample  to  settle  for  45  minutes.  After  45  minutes, you should run a glass or plastic rod gently down the inside of the cone and turn it to loosen solids clinging to the sides. Settling should then continue for another 15 minutes. The depth of the solids in  the  bottom  is  then  read  from  the  scale  and recorded  as  milliliters  of  settleable  solids  per  liter of  wastewater. ACTIVATED  SLUDGE SETTLEABILITY TEST The settleability test is often used with all kinds of activated sludge plants to find the amount of solids  in  aeration  units.  The  results  help  the operator  to  decide  when  to  waste  sludge  and  to find the rate of sludge return. The activated sludge settleability test can be run in a 1,000 ml graduated cylinder or in any clear, widemouthed container. The  container  should  be  ruled  off  into  10  units such  as  centimeters,  milliliters,  or  inches.  The sample is poured into the cylinder or jar up to the top mark and allowed to settle. Readings are taken from  time  to  time  to  find  settling  rates.  The sample  should  be  allowed  to  settle  for  about  30 minutes. FIVE-DAY   BIOCHEMICAL   OXYGEN DEMAND  (BOD5) TEST The BOD5 test is the most important test for finding the polluting strength of a wastewater. It is  the  most  widely  used  way  to  check  how  the treatment   plant   is   running.   The   BOD5 test indirectly   measures   the   amount   of   organic material in the sample. Either grab or composite samples  may  be  used  for  this  test. NPDES permits often state that influent and effluent flow-proportional  composite  samples  be taken   for   the   BOD5 test.   Normal   domestic wastewater coming into the plant should be in a 200 to 300 mg/l BOD5 range.  The  effluent  must comply  with  the  plant’s  NPDES  permit. To  run  the  test,  the  amount  of  oxygen  is measured in a portion of diluted wastewater, and another  portion  like  the  first  one  is  stored  at  20°C for 5 days. The glass bottles shown in figure 10-1 are  used  for  this  test.  During  the  5  days,  the microorganisms  eat  the  organic  matter  in  the wastewater and use oxygen at the same time. At the  end  of  5  days,  the  amount  of  oxygen  con- sumed by the microorganisms times the dilution factor  of  the  sample  gives  the  sample’s  5-day BOD.   The   dilution   factor   is   the   number   of milliliters  of  dilution  water  added  to  a  given number  of  milliliters  of  sample. CHEMICAL   OXYGEN   DEMAND (COD) TEST Like  the  BOD5 test,  the  chemical  oxygen demand  (COD)  test  finds  the  amount  of  oxygen required  to  consume  the  organic  matter  in  a wastewater  sample.  The  COD  test  does  not measure  the  amount  of  oxygen  used  by  the microorganisms.  It  uses  a  strong,  chemical  con- centrated sulfuric acid silver sulfate solution. It is a good operating control test because the results can be obtained in as little as 1 hour. COD test results  are  equal  to  or  greater  than  BOD5 test results. The chemical used in the COD test attacks more organisms in the wastewater than the slower BOD5 organisms. BOD5 data can often be related to  COD  data  by  a  multiplying  factor.  For  in- stance,  the  200  to  300  mg/l  BOD5 of   normal 10-22



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