Grit Happens You Dont Know What You Are Missing

Grit Happens You Dont Know What You Are Missing

GRIT HAPPENS YOU DONT KNOW WHAT YOU ARE MISSING

Pat Herrick, Hydro International Wastewater Division Hydro International - Wastewater Division

2925 NW Aloclek Drive, #140 - Hillsboro, OR 97124 (503) 615-8130

ABSTRACT Operator dissatisfaction with grit removal systems is all too common. Design of grit removal processes has been labeled as inadequate and misunderstood. Conventional guidelines target removal of grit larger than 210 m while minimizing organic content. In fact, many wastewater treatment plants across the country find over 50% of their influent grit is smaller than 210 m. In addition to designing for inadequate removal based on size alone other factors contribute to grit system failure. Conventional design assumes that municipal grit settles like clean sand particles in clean water. Grit removal systems are traditionally based on settling velocities of perfect spheres of silica sand particles with a 2.65 specific gravity in clean water. In reality, wastewater grit is comprised of silica sand as well as asphalt, concrete and various other materials that do not have a specific gravity of 2.65. Further, grit particles are not all perfect spheres and finally, grit is exposed to fats, oils, greases, and soaps in the collection system which coats the grit and changes its settling velocity. Grit systems can work as intended when designed with an accurate understanding of the nature and characteristics of the grit arriving at the treatment plant and how this grit actually behaves in wastewater. Advancements in grit management technology now allow 95% capture of grit as fine as 75 m while producing a clean, dry product. KEYWORDS: Grit Removal System, Size Distribution, Settling Velocity, Specific Gravity, Shape, Agglomeration, Sand Equivalent Size (SES) CONVENTIONAL DESIGN Meeting regulatory requirements for treated effluent quality is the major focus for wastewater treatment facilities. Grit removal, since it is not a regulated constituent, has historically been treated as an afterthought. Yet, wastewater treatment plants can be significantly and negatively impacted by grit. Traditional design guidelines target removal of coarse grit particles while minimizing organic content. The conventional design criterion for grit removal systems, based on Metcalf & Eddy, WEF MOP #8, and other trade manuals, has historically been to target grit particles 210 m and larger, with a specific gravity of 2.65. The results of these design criteria are more likely to produce a product with low organic content in order to make it acceptable at a landfill than to eliminate the grit which passes through the removal system causing problems for the plant. Producing a low organic content grit is an important goal to keep in mind when designing a grit removal system, as organics create odor issues, as well as increasing volume and water content which can increase vector attraction and make the product unacceptable at a landfill. However, the primary goal of any grit removal system should be to minimize the grit which is causing abrasive wear on mechanical equipment and depositing in process basins throughout the plant. ABRASION & DEPOSITION Grit is a nuisance material which causes abrasive wear to mechanical equipment and in turn increases maintenance and operational costs while reducing equipment performance and useful life. Equipment such as primary sludge pumps, thickener feed pumps, sludge dewatering pumps, digester draft tubes and mixers, centrifuges, and process collectors and screws are especially susceptible. In addition to the abrasive effects, grit accumulates in processes throughout a plant reducing processing capacity and detention time and influencing circulation patterns. Velocities through most process basins are lower than design velocities in the collection system. If grit is not removed in the headworks, it will settle once it arrives in a process basin which has lower velocities and a fine particle deposit limit. Fine bubble aeration basins and digesters have a deposit limit of approximately 100 m (Wilson, Tchobanoglous, Griffiths, 2007). If grit particles as small as 100 m are not removed in the headworks


they will deposit throughout the process where conditions are conducive to settling, the grit then consumes tank volume and reduces process capacity. Processes such as fine bubble aeration basins, primary clarifiers and digesters are susceptible to grit deposition. The reduction in processing capacity can affect the plants ability to achieve process design goals such as; reducing methane production, reducing volatile solids reduction or impeding mixing effectiveness in digesters (Massart, OKelley & Neun, 2010); or increasing operational costs, such as horsepower requirements in aeration basins (Herrick, 2009). At most treatment plants, grit accumulations typically happen gradually and continuously. It often goes unnoticed until a process is completely overwhelmed and needs to be shut down to manually remove the deposited grit, which is a labor intensive and costly operation. In some instances, when a process must be taken off-line, the flow to the entire process train must be diverted. This requires building excess plant capacity to use as grit storage which can significantly increase the size, cost and footprint of the plant. GRIT COLLECTION TECHNOLOGIES Most headworks grit collection processes are sedimentary processes and therefore knowing the nature and characteristics of the grit arriving at the treatment plant can be critical for design. In terms of design, the most important criteria to be considered are an accurate size distribution and particle settling velocity. Gravity Sedimentation Basins, Aerated Grit Basins and Forced Vortex Grit Basins are the three common types of grit removal systems. In each of these processes gravity is the predominant force field. Forced Vortex Grit Basins take advantage of centrifugal and other rotary derived forces to promote grit removal, but these forces are at most equal to, and in most cases significantly lower than gravitational forces. Therefore knowing the settling velocity of the incoming grit is critical to properly design the process. Utilizing both size distribution and settling velocity distribution enables the designer to establish a removal efficiency target and determine the design cut point particle. If size distribution alone is considered the removal results can be significantly different than anticipated. Many grit removal systems do not perform as designed because the settling velocity of the grit was over estimated. Once collected the grit must be washed and dewatered in order to produce a clean dry product for landfill. All three steps of the grit removal system, collection, washing and dewatering, must be as effective as the collection device otherwise the overall system efficiency will suffer. A system approach to design is the most effective. It is not uncommon to see a sufficient volume of grit captured in the collection step, only to lose collected material back to the process in the washing and dewatering steps. A grit study performed at Fox Lake, Illinois showed that while the aerated grit basin removed 58% of total grit volume entering the plant, the cyclone/screw classifier washing and dewatering equipment only retained 17% of what it received (Griffiths, 2004, Boldt 2005). The loss of grit in the washing and dewatering step reduced the systems overall efficiency to only 10%. GRIT CHARACTERIZATION One reason that conventional grit removal systems often fail to achieve effective removal is due to the size and settling velocity of grit being over estimated during design. This occurs when generic design criteria are used instead of developing a set of grit characteristics specific to the wastewater flow being treated. Generic design criteria often assume that each grit particle is a sphere settling in laminar conditions in clean water. Grit behavior in wastewater is a complex phenomenon governed by numerous factors such as size, specific gravity, shape, tendency for agglomeration with other wastewater constituents and environmental factors such as wet weather influences. Size The conventional design criterion of 210 m removal has allowed passage of large quantities of grit into wastewater treatment plants. Figure 1 shows the size distribution of grit found from onsite studies at a number of plants around the country (Osei and Andoh, 2008). Considering size alone it can be seen in the below chart that in many plants 50% of the incoming grit is smaller than the conventional design cut point of 210 m. Therefore, based solely on size distribution, by design, grit removal systems often miss half of the incoming grit. Modifying design criteria to remove 90% of the incoming grit requires changing the design cut point to somewhere between 75-150 m, depending on the gradation of the native grit.


FIGURE 1. PARTICLE SIZE DISTRIBUTION FOR GRIT AT MULTIPLE WWTPS (Osei and Andoh, 2008)

Specific Gravity The conventional design criterion has allowed passage of a large quantity of slowly settling grit into wastewater treatment plants evidenced by larger grit particles often found downstream of the grit removal process. The larger material that passes must be accounted for based on criteria other than size. The assumption has been that grit settles like silica sand however municipal grit is comprised of a variety of materials. Wastewater grit is comprised of asphalt, limestone, concrete, slowly putrescible organics and various other materials that rarely have a specific gravity of 2.65. Table 1 below lists the specific gravities of various materials that are likely to be constituents of grit entering a wastewater treatment plant. As can be seen, none of the materials listed has a specific gravity as high as 2.65, and the average value on the listed materials is in fact only two-thirds as dense.

Compiled Particle Size Distribution from Treatment Plants

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10 100 1000 10000

Particle Size (micron)

Perc

ent F

iner

Tha

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Chicago (10/27/2004) Chicago (10/28/2004) Chicago (10/29/2004) Florida (Ormand Beach)

Florida (Iron Bridge WRF) Florida (Eastern WRF) Florida (Largo WWTP) Florida (St. Petersberg SW)

Florida (Three Oaks WWTP) New England (Hartford, CT) Various (Atlanta) Various (Baltimore)

Calumet City WRP (7/15/2003) Calumet City WRP (7/18/2003) Clearw ater, FL (Northeast Plant) Clearw ater, FL (Marshall St. Plant)

Clearw ater, FL (East Plant) Green Bay, WI Tampa, FL

Compiled Particle Size Distribution from Treatment Plants

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Particle Size (micron)

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Chicago (10/27/2004) Chicago (10/28/2004) Chicago (10/29/2004) Florida (Ormand Beach)

Florida (Iron Bridge WRF) Florida (Eastern WRF) Florida (Largo WWTP) Florida (St. Petersberg SW)

Florida (Three Oaks WWTP) New England (Hartford, CT) Various (Atlanta) Various (Baltimore)

Calumet City WRP (7/15/2003) Calumet City WRP (7/18/2003) Clearw ater, FL (Northeast Plant) Clearw ater, FL (Marshall St. Plant)

Clearw ater, FL (East Plant) Green Bay, WI Tampa, FL


TABLE 1. SPECIFIC GRAVITY OF LIKELY CONSTITUENTS OF GRIT (Reade Advanced Materials, 2010)

Specific Gravity of Various Materials Quartz Sand 1.2 Earth 1.4 Limestone 1.55 Granite 1.65

Clay 1.8 Red Brick 1.9 Sand, wet 1.92 Gravel 2 Asphalt 2.2 Concrete 2.4

There is evidence that industry acceptance of this data is growing. For example, at the East Bay MUD wastewater treatment plant in the Oakland, California area it was determined that the specific gravity of their influent grit ranged from 1.04-1.61 with an average of 1.35 (Borys, Gabb and Hake, 2002). Illustrating the impact of this, a 210 m particle with a specific gravity of 2.65 would settle at 2.39 cm/s at 15C (59F); whereas a 210 m particle with a specific gravity of 1.35 would settle at a rate of 0.62 cm/s at the same temperature. The difference is significant, with the higher specific gravity particle settling almost four times more rapidly than the other particle. This is an important fact considering that conventional grit collection devices predominately rely on gravity as the main acceleration force causing separation of the solids from the liquid stream. Shape Grit particles vary in shape, are rarely spheres and many plants have noted that much of their larger grit is flat. An angular particle will exhibit reduced settling characteristics, settling more slowly than a sphere as its drag coefficient is higher (Jimimez and Madsen, 2003). More conservative design settling rates can better ensure capture of such angular particles. Agglomeration While in the collection system, the grit particles are exposed to a variety of constituents including fats, oils & greases (FOG), soaps, scum, some chemical constituents and dissolved gasses can attach to the grit particles and alter the particles settling characteristics. Such wastewater constituents can be significantly influenced by the commercial and industrial influences within the collection system. Regional aspects, such as the local soils and geology, winter traction control regimes and the type and age of the collection system can also impact the nature of grit. Wet Weather During peak wet weather events grit volume entering the plant can be 20-40+ times higher depending on the peak to average flow ratio, as well as the age and type of collection system (Wilson, Tchobanoglous, Griffiths, 2007). As much as 70% of the annual grit load can be received at the plant during a handful of first flush events (Wilson, 1998). These peak grit production periods frequently overload facilities, and must also be accounted for in the design of a grit removal system, especially in terms of grit conveyance and transport systems. GRIT SAMPLING Grit size distribution and settling velocity is not easy to characterize. First, there is no industry standard method for measuring grit. Obtaining representative and repeatable samples is difficult because grit does not flow evenly into the plant. It tends to travel in a higher concentration at the bottom of the channel, volume fluctuates with diurnal flow variations and grit volume significantly increases during wet weather events according to the energy/velocity profile in the collection system. Due to these significant variations sampling should occur over several days and ideally also include a wet weather event. Care must be taken in collecting the sample to ensure it is representative of incoming grit. Sampling from a column of flow or well mixed area ensures a representative sample is collected. A sufficient amount of material must be collected for analysis and collected grit volume should be correlated to incoming flow so an inlet concentration can be determined, indicating whether the sample is representative. While collecting grit, the holding vessel must be quiescent, encourage collected grit to settle and operate with a


low surface loading rate to ensure fine and slowly settling grit particles are retained. Once sampling is complete, the size distribution and effective settling velocity or effective specific gravity of the grit sample must be determined. Both characteristics are needed in order to have accurate data upon which to base a system design. In the absence of site specific information, it is advisable to err on the side of conservatism. In most cases, design should be based on the smallest practicable particle size which would typically be in the 75-106 m size range. Figure 2 below shows the size distribution of grit from various plants versus the settling velocity of the grit particles expressed in sand equivalent size or SES (Wilson, Tchobanoglous, Griffiths, 2007). SES expresses the measured grit settling velocity in terms of the size of sand sphere having the same settling velocity. SES is a method for normalizing all factors impacting settling velocity, such as size, shape, SG and agglomeration, to a known design point. The chart below shows that 106 micron is a convergent point where shifts in settling velocity, carry velocity, particle impacts, etc. merge into a sensible design point. Relating to Figure #1, it can be seen a 100 micron cut point targets most of the grit entering the plant. FIGURE 2. SIZE DISTRIBUTION VS. SETTLING VELOCITY DISTRIBUTION AT MULTIPLE WWTPS

Lacking an adequate grit characterization, as is always the case in new construction, a design cut point of 75-106 micron will generally remove 80-90% of the grit entering the treatment plant. Certain areas of the country are known for fine grit or sugar sand in these areas, or regions that contain loess, the finer cut point of 75 micron should be used. Most other areas of the country will achieve adequate results with a 106 micron design cut point particle. Reducing the design cut point particle effectively increases the surface loading requirement four-fold allowing sufficient settling time to remove the fine and light grit that previously has been overlooked, while the larger, heavier grit, that is transported to the treatment plant during higher flows seen during diurnal flow cycles, seasonal variations and spikes in flow from wet weather events, is easily captured. SYSTEM DESIGN Several factors should be considered when designing a grit removal system starting with a full characterization of the endemic grit including grit concentration, size distribution and settling velocity or effective SG. Understanding the actual characteristics of grit at a particular plant facilitates proper selection of the size and type of grit removal system that is required. With good data on the endemic grit, a cost benefit analysis can be determined, evaluating grit removal efficiency as compared to cost. Where specific data on endemic grit is not available a design cut point in the range of 75-106 micron will


generally provide the plant with adequate protection by removing 80-90% of the grit entering the treatment plant. Other considerations in this cost analysis would include upstream screening requirements, maintenance requirements, available space, and anticipated headloss/hydraulic gradient through the process and the level of protection to be afforded to downstream equipment and processes. Table 2 outlines important recommended guidelines to use when designing or evaluating a grit removal system. TABLE 2. GRIT REMOVAL SYSTEM CRITICAL DESIGN FACTORS

Design Guideline Define Design Requirements:

Grit Particle Size Analysis Settling velocity or Specific Gravity Required System Removal Efficiency Screening Requirements

Required Downstream Protection: Biological Processes Sludge Processing Equipment

Evaluate Equipment: Removal Efficiency/Performance Equipment Design/Features Space Requirements Headloss Requirements Cost: Capital, Installed, Operational Maintenance Requirements

EMERGING TREND Headworks screening and grit removal is the primary protection for all treatment processes and equipment in a wastewater treatment plant, yet it has been the most neglected part of the plant. To improve solids removal screen openings on influent screens have trended progressively smaller over the past 10-15 years. Years ago screen openings were frequently 1 and larger. Today, screens are commonly supplied with openings. It is logical that improving grit removal processes to effectively remove incoming grit is becoming a higher priority in plant designs. Biological processes have evolved to become more efficient and produce a cleaner effluent in progressively smaller areas. As plants move toward higher performing processes, effective grit removal becomes more important. The growing acceptance of Membrane Bio-Reactor (MBR) technology brings the need for advanced grit management systems into consideration. MBR technology requires extensive screening pretreatment which protects the membrane and often allows elimination of primary clarification. Without the protection of primary clarification, advanced grit removal should also be part of effective MBR pretreatment system design (Andoh 2009). Grit entering a MBR plant can cause abrasive wear and damage the membranes, which is often the most expensive equipment in the plant. Fine Bubble aeration is not a particularly advanced treatment process yet when a plant upgrades from coarse bubble aeration to fine bubble aeration, and in the process eliminates their primary clarifier, the impact of grit deposition increases for two reasons. First, the velocity in the aeration basin is reduced as is the deposit limit, making it the first likely location in the process train for grit to accumulate. Secondly, the deposited grit creates new maintenance challenges as diffusers cover the full floor of the basin which restricts the ability to clean them and makes the cleaning process more operator intensive and expensive. Diffusers covered with grit are less effective and additional horsepower may be required to achieve desired results. Today many plants are requiring increased effectiveness of their grit removal systems and their design engineers are taking a closer look at the size and settling velocity distributions of grit entering these plants. Newer editions of design manuals are recommending targeting particles smaller than 212 m in grit system design. In 2009, WEF MOP #8 issued a new chapter on Grit Removal stating that in 2008, a


WEF member survey reported grit density ranged from 1100 to 2200 kg/m3 (70 140 lb/ft3) and averaged 1,400 kg/m3 (90 lb/ft3). Metcalf & Eddy states that the specific gravity of clean grit reaches 2.7 for inerts but can be as low as 1.3 when a substantial amount of organic material is agglomerated with inerts. Bulk density and specific gravity of wastewater grit is lower than these factors are for clean silica sand indicating a shift in design considerations. Grit is not clean sand and many grit removal systems being designed today are targeting removal of particles in the 75-106-150 m size range in order to remove the particles that cause abrasive wear and deposit throughout the process. CONCLUSION Many installed grit systems fail to keep depositable grit out of the plant. In fact, they fail to remove the sizes and amounts of grit they were designed to capture. Grit system failure happens primarily due to a faulty assumption that municipal grit behaves like clean silica sand particles in clean water. The failure of many traditional grit removal systems has led to the misconception that grit removal systems cannot work, and that the only option is to deal with grit deposits in process basins downstream of the headworks and to deal with the abrasive wear from grit, both increasing maintenance and operational budgets. In order to design an effective system, design guidelines should be more comprehensive examining size and settling velocity of endemic grit, impact grit will have on downstream equipment and processes, grit system efficiency, headloss, space, cost, etc. A clear understanding of the grit entering the plant includes grit concentration, size distribution, effective specific gravity and/or settling velocity. Only with a clear understanding of the material to be removed can a system be designed to achieve specified results. When a complete characterization of the endemic grit is not available a conservative approach can provide effective grit removal. Targeting particles in the 75-100 micron range will typically remove 80-90% of the grit entering the treatment plant. Effective grit management requires a system approach. All components of the system must be effective in order for the overall system to yield the desired results. Improving grit collection only to lose a major portion of it back to the process in the washing and dewatering step is detrimental to overall removal efficiency and reduces the value of the entire system. Capturing a high percentage of the incoming grit load along with a high concentration of organics yields a product that is both difficult and expensive to landfill and can starve downstream biological processes. Each step of the process is important and should be optimally designed. Grit systems can work as intended when designed with an accurate understanding of the nature and characteristics of the grit arriving at the treatment plant and how this grit actually behaves in wastewater. An effective system addresses size as well as settling velocity or SG and produces a clean, dry product for landfill. The result is abrasive wear on equipment and deposits and accumulations in the plant are minimized. In turn, money is saved on maintenance and operational costs and equipment and processes perform better. REFERENCES Wilson, G., Tchobanoglous, G., and Griffiths, J. (2007) The Grit Book. Eutek Systems, Inc.: Hillsboro, Oregon. Massart, N., OKelley, S., Neun, G. (2010) Sustainable Biosolids Handling Digester Cleaning, Florida Water Resources Conference. May 2010 Herrick, P. (2009) A Portable Solution for Degritting Aeration Basins. Pollution Equipment News. January 2009, pp 17-18. Griffiths, J. (2004) Fox Lake Regional WRF Grit Testing Results. Grit Solutions. May 7, 2004 and August 19, 2003. Boldt, J. (2005) Eliminating Grit Deposition Problems through Objective Grit System Design, A Case Study at the Fox lake NRWRP Fox Lake, IL. Illinois WEA Conference. January 2005.


Osei, K. and Andoh, R.Y.G, (2008) Optimal Grit Removal and Control in Collection Systems and at Treatment Plants, World Environmental and Water Resources Congress, Honolulu, Hawaii, 12-16 May. Boyrs, A., Gabb, D. and Hake, J. (2002) Performance Evaluation of Aerated Grit Chambers and Proposed Modifications to Increase Grit Removal Efficiency at East Bay Municipal Utility District WWTP. Conference Proceedings from California Water Environment Association Annual Conference, April 4. Session 22. Jiminez, J. and Madsen, O. (2003) A Simple Formula to Estimate Settling Velocity of Natural Sediments. Journal of Waterway, Port, Coastal and Ocean Engineering, ASCE March/April 2003 Andoh, R.Y.G. and Neumayer, A. (2009) Fine Grit Removal Helps Optimize Membrane Plants. WaterWorld. January 2009, pp 28. Reade Advanced Materials. Weight Per Cubic Foot and Specific Gravities. Reade Specialty Chemical Resource Company. http://www.reade.com/Particle_Briefings/spec_gra2.html (Accessed February 2010). Water Environment Federation and ASCE (2009) WEF Manual of Practice 8, 5th Edition, Alexandria, VA, WEF, ASCE. Metcalf & Eddy, Inc, (2003) Wastewater Engineering: Treatment and Reuse, 4th ed., McGraw-Hill.

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Grit Happens You Dont Know What You Are Missing