Chapter 9  Solids                                                                                   return to Table of Contents

                        Total Solids (Residue)

                        Settleable Solids

                        Suspended Solids

                        Total Sludge Solids (Volatile and Fixed)

                        Turbidity

         Solids may be insoluble and suspended in the water or they may be dissolved in the water.  Testing for the type and amount of solids provides information necessary for treatment adjustments.  The amount of organic matter present indicates the work load for biological treatment.  The amount of colloidal solids shows the pretreatment efficiency.

          Total solids consists of dissolved and suspended solids.  Suspended solids can be caused by the presence of suspended matter such as mud, silt, finely divided organic and inorganic matter and microscopic organisms such as algae.  Suspended solids are composed of two parts: settleable and nonsettleable.  The difference between them depends on the size, shape and density of the particles.

          The suspended solids determination is one of the major parameters used to evaluate the strength of waste waters and the efficiency of treatment units.  Both the total and the volatile suspended solids tests are used.  The measurement of suspended solids begins with separating those solids that can be removed from suspension in the water by filtration.  These solids contain organic and inorganic materials.  The measurement of the organic materials is referred to as “volatilization”.  This is accomplished by a combustion procedure in which the organic material is converted into water and carbon dioxide by controlled temperature.  The remaining material is considered “fixed” or “inorganic”.

          A suspension of tiny particles in some medium is called a colloidal dispersion, or a colloid.  The suspended particles are single large molecules or aggregates of molecules or ions ranging in size from 1 to 1,000 nanometers.  Because the colloidal particles all have an outer layer of ions with the same charge, they repel each other and do not easily aggregate to form particles that are large enough to precipitate.

          The destruction of a colloid, called coagulation, can usually be accomplished by heating or by adding an electrolyte.  Adding an electrolyte neutralizes the adsorbed ion layers, now the particles can aggregate.  The key to effective coagulation and flocculation is an understanding of how individual colloids interact with each other.  Turbidity particles range from about .01 to 100 micrometers (microns) in size.  (micro means 10^-6 and nano means 10^-9). The larger fraction is relatively easy to settle or filter. The smaller colloidal fraction (from .01 to 5 microns) presents the real challenge.  Their settling times are intolerably slow and they easily escape filtration.

          The behavior of colloids in water is strongly influenced by their electrokinetic (temperature causes motion) charge.  Each colloidal particle carries a like charge, usually negative.  This like charge causes adjacent particles to repel each other and prevents effective agglomeration and flocculation.  As a result, charged colloids tend to remain discrete, dispersed and in suspension.

          On the other hand, if the charge is significantly reduced or eliminated, then the colloids will gather together.  First forming small groups, then larger aggregates and finally into visible floc particles which settle or float rapidly and filter easily.

          One treatment process uses acid to change the charge on the colloids then adds long chain high molecular weight polymers of both positive and negative charges in a rapid mix to form coagulated collections.  Then, just as the mixture enters a rapid deceleration zone, very tiny air bubbles are injected.  These bubbles get trapped in the coagulant, start increasing in volume (decreasing the density of the mass) and a flocculation literally makes the suspended solids jump out of the water.

Types of colloids      

Familiar Examples	Dispersing Phase	Dispersed Phase	Colloid System
Fog, aerosol sprays	Gas	Liquid	Aerosol
Smoke, airborne bacteria	Gas	Solid	Aerosol
Whipped cream, soap suds	Liquid	Gas	Foam
Milk, mayonnaise	Liquid	Liquid	Emulsion
Paint, clays, ink	Liquid	Solid	Sol1
Marshmallow, polystyrene foam rubber	Solid	Gas	Solid Foam
Butter, gelatin, cheese	Solid	Liquid	Solid emulsion
Ruby glass	Solid	Solid	Solid sol

 When a colloid is made by mixing particles of one substance with those of another, the mixed-in substance is called a dispersed phase and the containing substance the dispersing phase. The properties of the mixtures arise from the size of the particles in the dispersed phase and from the way the particles respond to the constant bumping they give each other.²

          The term turbidity is simply an expression of the physical cloudiness of water.   The measurement of turbidity is another important test.  The Safe Drinking Water Act stipulates specific monitoring requirements for turbidity.  Turbidity should be measured in a sample as soon as possible to obtain accurate results.  The turbidity of a sample can change after the sample is collected.  Shaking the sample will not recreate the original turbidity.  Continuous instream sensing of turbidity is common.

          One of the major purposes of the wastewater plant is to make a substantial reduction in the solids load of the wastewater.  The type and distribution of solids in the plant influent and effluent and at various stages within the plant are indicators of plant efficiency.  EPA refers to solids as residues.  The important fractions of solids include the following:

Fraction	EPA name	Temperature
Settleable Solids	Settleable residue	Ambient
Total Solids (TS)	Total residue	104 ± 1º C
Total Suspended Solids (TSS)	Nonfilterable residue	104 ± 1º C
Total Dissolved Solids (TDS)	Filterable  residue	104 ± 1º C
Total Volatile Solids (TVS)	Volatile residue	180º C
Total Volatile Suspended Solids (TVSS)		550 ± 50º C
Total Volatile Dissolved Solids (TVDS)		550 ± 50º C
Ash	Fixed residue	550 ± 50º C

                                      

          By and large, solids analysis is inexpensive but it can be time consuming. Weigh, separate; weigh, separate;  and weigh, separate again.  The overall idea in this type of analysis is to separate the solids from a weighed portion of the wastewater, then determine the isolated solids content by weighing.  For the TS (total solids) fraction this requires a simple pre-weighing of the sample and a container, evaporation of all the water in the sample by placing it in a 104^º C oven and then re-weighing the container and the residue.  The difference is the TS.  Many minutes pass by performing this simple lab procedure.  Planning ahead will allow the technician to squeeze in other tests while waiting for this procedure to complete staging.

          In theory TS = TDS + TSS, however this seldom works out, because of the way the analyses are performed.  The fractions are separated by filtration, the TSS fraction being all the material retained by a glass fiber filter with a nominal pore size of 2 lm while the TDS fraction is all the material passed by the filter.  The TS and TSS fractions are dried at 104^º C while the TDS fraction is dried at 180^º C.  Adsorbed and bound water is almost completely removed at 180^º C, while the 104^º C drying removes almost no bound water.

          The volatile parameters are always obtained by the difference between the dried portion and the ash.  .  The solids are composed of inorganic and organic materials and the volatile portions are interpreted to represent the organic fraction while the ash is supposed to be all the inorganic material.  This is an oversimplification of the actual results of these analyses as many inorganic materials are lost or changed in mass during the 550^º C ash procedure.  However, this does not really detract from the use of these parameters as process monitoring tools.

                 The calculation for TSS, TS and TDS is straightforward if the weight is determined in grams and the sample size in mL.  .

                            [If the weights are expressed in mg and the sample volume in L then there is no multiplier (i.e., the 1,000,000)]

                 The TVS, TVSS, and TVDS fractions are determined by the difference between the TS, TSS, and TDS values, respectively, and the ash value obtained from ignition of the respective solids fraction at 550 ± 50^º C for an hour.  Needless to say, if the volatile part of a fraction is to be determined, the filter or crucible should be ignited initially to obtain the proper tare value.

 

             or  

 

          All solids analyses require a minimum residue of at least 10 mg material on the filter or in the crucible to count as a detect.

 

TSS checklist (2540 D, SM₁₈)

 

                         Procedure

          ___  1.  Wash glass fiber filter with three 20 mL portions of reagent

water.

          ___  2.  Dry filter in oven at 104^º C on a planchet or watchglass for at

least 1 hour.

          ___  3.  Place filter in desiccator and allow to cool for no more than 10

minutes.

          ___  4.  Check calibration on analytical balance, set to zero and

determine weight of filter.  Record the weight on the benchsheet.

          ___  5.  Assemble filter funnel with glass fiber filter and pre-wet filter

with a small portion of reagent water.

          ___  6.  Mix the sample well, then measure the needed sample size

into a graduated cylinder.  Record the sample size and identification on the benchsheet.

          ___  7.  Pour the sample into the filter funnel and apply the vacuum.

Rinse the contents of the graduated cylinder into the filter funnel with two 10 mL portions of reagent water.  Wash the material adhering to the side of the filter funnel down onto the glass fiber filter with additional reagent water, if necessary.

          ___  8.  Transfer the filter to a watchglass or planchet, place in the

oven and record time and oven temperature on the benchsheet.

          ___  9.   At the end of an hour, transfer the filter to a desiccator and

allow to cool for no more than 10 minutes.  Weigh the filter on the analytical balance and record the weight on the benchsheet.

          ___ 10.  Replace the filter in the oven, dry for an hour, cool in a desiccator and re-weigh the filter.  If the weight difference between the first and second weighing is less than 4 %, record the final weight.  Otherwise, continue the drying, cooling and weighing cycle until less than 4 % difference is obtained.

          ___ 11.  Calculate the TSS and the RPD.  Calculate RPD

                 (Relative % Difference) and update the control chart.

 

 

Question for Chapter 9

 

   1.  The total solids test measures all of the solids in a sample, some of  which are __________ and the

         remainder are __________.

 

   2.  Of these two forms of solids, some are __________ and some

         are __________ matter.

  

   3.  The amount of __________   __________ in a wastewater indicates the work load for biological treatment.

 

   4.  

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  ¹A word coined in 1899. The most important colloidal systems are those involving a solid dispersed in a liquid, and such systems are called sols.          

²From Compton's Interactive Encyclopedia © 1998 The Learning Company, Inc.