Standardized Rating Needed for Municipal Grit Systems

By Dr. George Wilson

Over three decades ago, WEF completed procedures for standard rating of aeration equipment. By providing the industry a procedure whereby they could rate the oxygenation capacity of equipment under specified standard conditions, the industry was given a level playing field on which to evaluate technologies.

Benefits went beyond that; they provided procedures that allowed engineers to confidently convert standards to required oxygenation capacities under actual operating conditions in order to determine the aeration equipment requirements.

Unfortunately, the objective measurement of grit has, until recently, not been available. There is no standard method for sampling and measuring grit. Without an objective measurement procedure, one cannot specify Grit Removal Systems (GRS) that can successfully resolve grit related problems.

A similar benefit to the industry is possible by following the same two-step procedure that is used for specification of aerations equipment:

1. Determine the efficiency of the overall GRS using fine sand in tap water at a specified temperature.

2. Determine the GRS requirements to achieve the desired degree of grit removal based on a site-specific grit characterization measurement.

Process Mass Balance

In the case of a treatment process, the objective basis for a valid measurement procedure is a mass balance around the conserved quantity of concern. For a GRS, the conserved quantity is the Fixed Solids (FS). Thus, for valid measurement of a GRS, mass balance must be achieved.
lbs FS in = lbs FS out + lbs FS removed

Key Objectives

Although there is a standard method for determining Fixed Solids, obtaining a valid measurement of grit depends on achieving a representative sample. Current procedures for sampling suspended solids in flowing wastewater will not provide representative samples. However, by using sampling procedures appropriate to the characteristics unique to flowing sediments, FS mass balances are achievable.

Rating Procedures for GRS

To obtain a standard grit removal rate, several conditions must be met in addition to the use of valid sampling procedures. The GRS must be tested as a complete system, including grit separation, washing and dewatering stages, using tap water and F110 Silica Sand. Five sampling points, continuously measured for total and sampled flow, must be used to determine the standard rating for the system:

• Separator influent and effluent
• Washing unit influent or Separator underflow
• Dewatering unit effluent
• Dewatered grit

Test results must be reported as % influent sand mass removed as dewatered sand larger than 100-microns. Mass balance must be demonstrated to insure that a valid and objective system rating has been measured.

Grit Characteristics

Characteristics of grit in the wastewater stream must be objectively measured. With this information, a standard rating can be applied in the specification of equipment needed to meet the grit removal requirements. To do this, both the wet-sieve size and the settling velocity of the grit size fractions must be assessed. It must be kept in mind that it is not the size but the settling velocity of the grit particle that determines the time required for the particle to reach the separator base.

Within the past two years, numerous measurements of the settling velocity of wet sieved grit size fractions have been made at plants throughout the country. The results have been consistent yet totally unexpected. Above a grit size of 100-microns, the median settling velocity of wet sieved grit size fractions do not change significantly through an 840-microns particle size, but remain constant and independent of size.

Settling Characteristics As a Basis of Design

Prior to the objective measurement of influent grit characteristics, a rational procedure for determining the desired grit removal cut-point particle size was to obtain a dry-sieve size distribution of a burned grit sample. While this would seem a reasonable approach, it usually fails to solve grit deposition problems.

In order to prevent most grit deposition, grit larger than 100 microns in size should be removed by the GRS. But the settling velocity of grit is usually less than that of a sand particle equal in size to the grit particle. As raw sewage moves grit through a collection system, a thin coating of greases is deposited on grit particles. This coating adds volume but little weight, thus reducing mass and specific gravity. Consequently, grit particles do not settle at the same velocity as the clean sand particles of identical size. Thus, a 100-micron size grit particle may have the settling velocity of a 60-microns sand particle (60-microns "Sand Equivalent Size", SES). In order to remove all grit larger than 100 microns, it would be necessary to provide enough settling time in the GRS to settle a 60-micron sand particle.

One must measure the settling velocity distributions of wet-sieve size fractions of accurately sampled grit in order to assess the settling time required to remove all grit larger than 100-microns size. Only in this way can grit deposition be eliminated.

About the Author:
Dr. George Wilson received his B.S., M.E., and Ph.D. in Civil Engineering from the University of California, in Berkeley, CA. He is the President and founder of Eutek Systems Inc., a WWEMA member company and manufacturer of engineered municipal grit systems.

Note: The views expressed in this article are the views of the author and do not necessarily reflect the views or policies of WWEMA