Environmental Express Joe Boyd Solids and BOD Analyses: Pitfalls and Troubleshooting
Solids Analysis Total Dissolved Solids SM 2540C, ASTM D5907-03, USGS I-1750-85 Total Suspended Solids SM 2540D, USGS I-3765-85 Settleable Solids SM 2540F Total Solids SM 2540B
Total Dissolved Solids (TDS) Method Summary: An aliquot of sample is filtered through a glass fiber disk. The filtrate is collected in a pre-weighed evaporating dish. The liquid is evaporated at 104 C and the dish then placed in an oven at 180 C. The dish and residue are weighed with the mass of the residue determined by difference.
Total Dissolved Solids (TDS) Equipment Specification Filter Evaporating dish Binderless Glass Fiber, 1.5µm pore size Platinum, porcelain, or silica glass; 100mL vol. Vacuum filtration device Variety of options Oven 104 C and 180 C Weight stability +/- 0.5mg of previous weight
Total Dissolved Solids (TDS) Potential problems with evaporation dish The lighter the better. Platinum is the lightest material but the most expensive. Typical weight of porcelain dish is ~ 80g. Porcelain is porous. High humidity can affect weight stability. Allow the same cooling / desiccating time each time the sample is weighed to achieve consistent results. A 1-hour wait time in the desiccator is generally sufficient. Obtain the initial weight of the dish immediately prior to use.
Total Dissolved Solids (TDS) Why two temperatures and why are they important? The initial evaporation step at 104 C allows sample evaporation without any splattering. Drying at 180 C drives off all mechanically occluded water and most of the water of crystallization. Samples of high sulfur content may retain more water than normal The 180 C temperature also drives off carbon dioxide and converts bicarbonates to carbonates. Too high of a temp may cause further conversion to oxides. Some organic matter may be lost at 180 C but the temperature is a good compromise between complete water removal and minimal volatile loss.
Total Dissolved Solids (TDS) What is the right sample volume for this test? Standard Methods states Choose sample volume to yield between 2.5 and 200 mg dried residue. The lower limit is 5x the allowed variance between weights giving sufficient weight differences between initial and final weights. The upper limit allows for a timely and complete removal of water from the crystals in the sample. Amounts greater than 200mg may layer in the dish trapping water. Typically 100mL is a good volume to start with but the method does not dictate what you must use.
Total Suspended Solids (TSS) Method Summary An aliquot of sample (no more than 1-L) is passed through a pre-washed, pre-weighed filter. The filter is then dried at 104 C. After drying the filter is re-weighed and the TSS is calculated.
Total Suspended Solids (TSS) Equipment Specification Filter Binderless Glass Fiber, 1.5µm pore size, prewashed and weighed Vacuum filtration device Variety of options Oven 104 C Weight stability of filter +/- 0.5mg of previous weight
Total Suspended Solids (TSS) Potential problems with the filter paper Preparation is the key Wash, dry, desiccate, weigh. Repeat dry through weigh steps until a weight-stable filter is achieved. Prepared filters such as the ProWeigh can solve filter preparation problems. New documentation for DoubleWeigh filters. Shows weight stability of filter for audit trail. Many auditors and auditing bodies now requiring this type of documentation.
Total Suspended Solids (TSS) Date/Time Box ID Identifier Weight 1 Weight 2 Lot # Sep 11 2013 04:38:26:743AM a00000 D5984 0.1245 0.1245 600006 Sep 11 2013 04:38:34:493AM a00000 D5985 0.1215 0.1215 600006 Sep 11 2013 04:38:43:243AM a00000 D5986 0.1195 0.1195 600006 Sep 11 2013 04:39:12:760AM a00000 D5987 0.1186 0.1188 600006 Sep 11 2013 04:39:21:260AM a00000 D5988 0.1223 0.1221 600006 Sep 11 2013 04:39:50:777AM a00000 D5989 0.1186 0.1187 600006 Sep 11 2013 04:39:59:277AM a00000 D5990 0.1182 0.1182 600006 Sep 11 2013 04:40:29:793AM a00000 D5991 0.1203 0.1205 600006 Sep 11 2013 04:40:38:543AM a00000 D5992 0.1214 0.1213 600006 Sep 11 2013 04:41:10:560AM a00000 D5993 0.1211 0.1214 600006
Total Suspended Solids (TSS) Filter Paper Continued Consistency in pore size 1.5µm is the industry standard pore size for filters used for TSS analysis. Once you analyze a sample for TSS using a filter with a particular pore size you MUST stick with that size. What filter has the edge? Frayed edges can cause loss of filter material during analysis resulting in biased results.
Total Suspended Solids (TSS) Filter Paper Continued Filters that stick to the pan indicate a high organic content. Wash the filter well with DI Water after sample filtration to remove any residue that is stuck to the bottom of the filter. Orientation of the filter There is an up and down side to the filters. Ensure they are oriented correctly prior to filtration.
Total Suspended Solids (TSS) Shake it up! The key to obtaining a representative aliquot of sample is to thoroughly mix the sample. Constant sample movement helps. Look for fine silt stuck to the bottom of containers. Remove debris that would not be considered part of a typical water sample such as sticks and leaves.
Total Suspended Solids (TSS) What is the right sample volume for this test? Try to limit the sample size to yield no more than 200mg of residue. Experience with the sample is your best guidance. Typical sample volumes are: Mixed Liquor: 1 10 ml Influents: 25 50 ml Effluents: 300 700 ml Storm water: 500 1000 ml Unlike TDS, Standard Methods dictates a sample volume of 1-L as the largest volume one can use.
Biochemical Oxygen Demand (BOD) Standard Methods 5210B A known volume of sample has its initial dissolved oxygen content recorded. After a five day incubation period at 20 C the sample is removed from the incubator and the final dissolved oxygen content is taken. The BOD value is calculated from the depletion and the amount of sample used.
Biochemical Oxygen Demand (BOD) Equipment / Reagents BOD bottles Incubator Dilution water Seed Standard DO Meter Specifications Hold 300mL, seal tight, no oxygen migration Hold temp of 20⁰C Contains the 4 required nutrients and is aerated Viable and active GGA with TV of 198mg/L Membrane electrode or optical luminescence
Biochemical Oxygen Demand (BOD) Bottles Glass or Plastic? Glass is traditional but can crack, chip, break, and are a pain to clean. If not cleaned properly can lead to poor results. Plastic bottles are designed for 1-time use no washing! Lighter, can fit more in an incubator. EPA approves both styles.
Biochemical Oxygen Demand (BOD) Dilution Water Choose your water type carefully. Distilled water is not always the best choice as certain metals and volatile organics may distill over. Deionized water is a popular choice but proper maintenance of your resin tanks is key. The oxygen depletion after 5 days must be < 0.2mg/L.
Biochemical Oxygen Demand (BOD) Seed Choosing your seed source can make or break your test. The safest route is generally to purchase a commercially available freeze dried seed. These are generally very consistent from one batch to the next and provide predictable results. If you have a source of seed material (i.e. wastewater treatment plant), research where you take your seed from. Should be a consistent part of the plant (NOT influent). Ahead of any disinfection area. Primary or Secondary Effluent are generally good places. A good Seed Correction Factor (SCF) should be between 0.6 1.0 mg/l.
Biochemical Oxygen Demand (BOD) Seed Continued Too high of a SCF Check your water quality to ensure no contamination. Check other labware such as bottles, pipettes, tubing, etc for cleanliness. If you are providing your own seed look for changes in the seed source that could affect the strength of the seed and make adjustments. If the GGA is within range increase the amount of rehydration water used in the commercial freeze dried seed or check your seed source and potentially dilute it with DI Water. This process is approximately linear (especially with the commercial seeds) so if you need to reduce your SCF by 20% increase the rehydration water by 20%.
Biochemical Oxygen Demand (BOD) Seed Continued Too low of a SCF Check your water quality to ensure no contamination. Check other labware such as bottles, pipettes, tubing, etc for cleanliness. If you are providing your own seed look for changes in the seed source that could affect the strength of the seed and make adjustments. If purchasing a commercial freeze dried seed reduce the amount of rehydration water by the same factor you want to increase your SCF.
Biochemical Oxygen Demand (BOD) Standard The standard used for BOD analysis is Glucose / Glutamic Acid, better known as GGA. This is made by adding 150mg of glucose and 150mg of glutamic acid to distilled or DI Water and diluting to 1-L. The true value of the GGA standard is 198 mg/l of BOD. As this is a demand standard there is an acceptable range defined by the method of +/- 30.5 mg/l. If making this standard up in the lab it is important to desiccate the glucose and glutamic acid prior to weighing out so it does not absorb moisture which can affect the weight. Commercially available, premade vials of GGA are available.
Biochemical Oxygen Demand (BOD) Standard Continued If your SCF is within range but your GGA results are high, decrease the amount of seed in your GGA and/or samples by 1-2 mls. If your SCF is within range but your GGA results are low, increase the amount of seed in your GGA and/or samples by 1-2 mls.
Biochemical Oxygen Demand (BOD) The biggest issue with the BOD test, however, is that the holding time for samples is 48 hours and the test takes 5 days. By the time you find out if your test is valid it is too late to do anything about it. One must give the utmost care to each step of your procedure to avoid the possibility of invalid data.
Questions? Joe Boyd Environmental Express joeb@envexp.com 800-343-5319 December 26, 2012