Grit vs. Grit. A Pilot Evaluation Comparing Two Grit Removal Technologies Brian F. McNamara1*, Thomas Kochaba2, Jimmie Griffiths3 David Book3 1
Hampton Roads Sanitation District HDR Engineering, Inc. 3 Grit Solutions *
[email protected] 2
ABSTRACT The Army Base wastewater treatment plant (ABTP) is a 68 ML/d (18 MGD) secondary wastewater treatment facility. ABTP is currently undergoing design for advanced biological nutrient removal. The design will install a new pre-treatment facility (PTF) with grit removal. The owner desired to improve the grit removal facilities as the original grit channels were ineffective. The design of the new PTF is space constrained for the given site. Manufacturers of a gravity settling unit and a forced vortex unit were contacted to conduct a side by side pilot test. The goals of the study were to evaluate: manufacturers expected efficiencies, the grit removal efficiencies over a full range of grit particle sizes from 100 µm to 300 µm, and compare efficiency of removal with cost of grit removal unit. Based on the pilot testing results, the municipality is proceeding with full scale designs for the gravity settling unit.
KEYWORDS:
Grit efficiency, vortex, gravity settler, grit sampling.
INTRODUCTION Gravity Settling Technology Review Grit separation technologies using gravity are typically velocity channels or detritors. Velocity channels are inefficient and detritors require a lot of area. HRSD investigated a gravity settling unit that employs a series of stacked conical trays, and has a relatively low head loss. See Photo 1 for a full scale unit. Raw influent (RWI) flow is introduced into a splitter box and equal portions of the flow are sent to each tray. The stacked trays provide a large total surface area, and a small unit process foot print. The full scale unit would employ 9 trays with a diameter of 3.66 m (12 ft). The 9 tray unit is placed inside a deep rectangular concrete tank. Flow enters tangentially onto each tray (Figure A). The effluent flow of each tray exits the perimeter and center of each tray, travels upward and overflows at the top of the unit. The final design would employ two units, each with a hydraulic capacity of 68 ML/d (18 MGD), and a bypass channel. The trays are made of thick plastic and stainless steel supports. The concrete tank would be lined with a PVC coating for corrosion protection.
Grit vs. Grit
Photo 1
Figure A Stacked Gravity Trays
Forced Vortex Technology Review Medium energy forced vortex grit separation is very popular and HRSD operates four treatment plants with vortex units. Vortex grit separators have a small foot print, low head loss, and operate over a wide range of flow rates. Flow tangentially enters a circular chamber and begins rotating. The flow at the perimeter of the vortex is faster relative to the flow at the center of the vortex. The low velocity at the center of the vortex allows grit particles to drop out and separate from the bulk fluid. “The size of the settlement chamber is determined by the rate of flow to the plant and dimensioned to allow time for the grit entering the grit trap (vortex unit) to settle under gravity” (Willmott 2001). The vortex design chosen by HRSD uses an interior baffle. Flow enters the chamber between the perimeter wall and the interior baffle. The flow must navigate under the baffle and then overflow to an effluent surface trough. This design helps to deter any grit bypassing through the unit. Figure B represents the vortex technology investigated by HRSD. The final design would employ two vortex units with a diameter of 6.7 m (22 ft) and no bypass channel. The interior of the concrete tank would be protected with a PVC coating for corrosion and the interior baffle would be made of stainless steel. Gravity Pilot Unit The gravity settler grit pilot unit employs two stacked conical trays with a diameter of 1.22 m or 2.34 m2 (4 ft dia. or 25 ft2 area). Raw influent (RWI) flow is introduced into a splitter box and equal portions of the flow are sent to each tray. The Product representative stated that the optimum operating parameters were 643 Lpm (170gpm) to achieve a 90% removal at a cut point of 75µm of grit. Cut point is an efficiency term. Cut point is the minimum grit particle size that a system will capture at a given flow rate. It implies that all particle sizes larger than the minimum will also be captured. It is usually quantified at a specific gravity (usually 2.65) and as a percentage.
Grit vs. Grit
Vortex Pilot Unit The forced vortex pilot unit employed a tangential RWI feed and an effluent overflow arrangement (Figure B). The diameter of the vortex unit was 1.22 m (4 ft). Optimum operating parameters for the vortex unit were 681 Lpm (180 gpm) to achieve a cut point of 106µm with 95% removal.
Figure B Forced Vortex
METHODOLOGY Sampling & Analysis Grit influent and effluent sampling was accomplished by using a vertical sampler in open channel flow (Photo 2 & 3). The sampler consisted of a slotted 0.15 (six inch) PVC pipe and a portable pump (Photo 4). The dimensions of the sample slot were proportional to the flow velocity, channel depth, and pump draw. The dimensions were designed to match the pump withdrawal rate with the velocity flow in the channel. Thus only the grit traveling in line with the sample slot would enter the vertical pipe, and grit not in line with the sample slot would go around the sample pipe. A portion of the sample pump discharge was sent to a 50 µm settler. The material captured in the 50 µm settler was processed for further analysis.
Grit vs. Grit
Photo 2 Vertical Sampler
Photo 3 Dual Sampling
Photo 4 Portable Pump Testing protocol used two vertical samplers in the same RWI channel (Photo 3). The vertical samplers were spaced 0.46 m (18 inches) apart (center to center). One vertical sampler was dedicated to the influent of the gravity settling unit. The other vertical sampler was dedicated to the influent of the vortex unit. Grit effluent samples were collected by diverting a portion of the effluent flow to a 50 µm settler. Samples were collected from the RWI, and grit effluent of each unit. Sampling was continuous and lasted over a four hour period for the first test and a six hour period for the second test. Influent and effluent samples were processed through a wet-sieve to separate the different particle sizes (Photo 5 & 6). The sieved particles were then analyzed for fixed solids. Laboratory data, in conjunction with sample flow rates were used to calculate grit concentration and grit mass flow rates. Removal efficiencies for each range of particle size were calculated for each unit at the designated hydraulic loading.
Photo 5 Stacked Sieves
Photo 6 Sieved Grit
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Efficiency Calculation The model for removal efficiency is displayed as Figure C. Only influent and effluent sampling were conducted for each pilot unit. The final or captured product was not analyzed. The removal efficiency was calculated as follows: % Grit Removed = Grit Influent Mass – Grit Effluent Mass x 100 Grit Influent Mass This standard calculation was performed for each micron range of sieved grit particles.
Figure C Grit Sampling Locations
Grit vs. Grit
Test Protocol The units were setup side by side and received the same RWI flow at the same time (Photo 7).
Photo 7 Vortex Left and Gravity Settler Right The test protocol was to simulate the proportional full scale 68 ML/d (18 MGD) plant flow to each unit. A scale flow rate of 68 ML/d was not employed due to the capacity limitations of the sample pumps and the site constraints (6.1 m lift from the RWI channel). Accordingly target scale flow rates were adjusted to 38 ML/d (10 MGD) and a high hydraulic flow of 57 ML/d (15 MGD). Testing was conducted for two days. The design efficiencies of the gravity pilot unit are calculated based on a grit specific gravity of 2.65. The manufacturer stated removal efficiencies to 90% at 643 Lpm (170 gpm) for 75µm. The key parameter for calculating scale up was the surface overflow rate (SOR). The pilot feed rates and predicted grit removal cut points are listed as:
Feed Rate Lpm (gpm) 643 (170) 935 (247)
Cut Point 75µm 100µm
SOR L/m2-day 402,837 575,453
Comparable Plant ML/d 40 57
Full Scale No. of 3.6 m trays Required 9 9
The equation for hydraulic ratios between pilot and full scale of the vortex was given by the manufacturer as: (Diameter Full Scale/Diameter Pilot)2.2 x Pilot Flow Rate = Full Scale Flow Rate
Grit vs. Grit
The vortex pilot testing protocols were as follows: Feed Rate Lpm (gpm) 681 (180) 935 (247)
Cut Point
Comparable Plant ML/d
Predicted Removal at 2.65 sg
106µm 150µm
42 57
95% 95%
Pump feed rates to each pilot unit were modulated by the engine throttle. Flow rates were verified by fill test to each pilot.
RESULTS & DISCUSSION The pilot test results are displayed graphically as Figures D & E for the gravity settler and Figures F & G for the vortex. The graphs display the kilograms of grit per million liters of wastewater for each grit particle size. Influent grit is shown in blue and effluent grit is shown in red. These graphs reveal the efficiency of grit capture by each pilot unit for each particle size range for each day of testing. Comparing the blue raw influent lines for Figures D and F on December 17th, there are only small differences in grit quantity. Given that the graphs represent two separate and simultaneous samples of the same raw influent stream, there appears to be good correlation for the results. The RWI profiles are repeated again on December 18 for Figures E and G. Because each pilot unit was sampled and analyzed separately, the efficiency can be examined in detail for each pilot unit. The relative magnitude and shape of the grit curves are similar for both RWI and FNE on all four figures. This illustrates that the results of the sampling and testing protocols are adequate for performance evaluation.
Grit vs. Grit
Figure D Gravity Settler 39 ML/d (10.4 MGD) December 17, 2007
Figure E Gravity Settler 57 ML/d (15.1 MGD) December 18, 2007
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Figure F Vortex 40 ML/d (10.5 MGD) December 17, 2007
Figure G Vortex 56 ML/d (14.8 MGD) December 18, 2007
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The grit removal efficiencies for each pilot unit are displayed on Table 1. The cut point of 105μm was chosen. Grit sizes below 105μm were not present in large quantities, and it was not expected to be a significant downstream process problem. Testing showed that at higher flow rates (57 ML/d or 15 MGD scale) the gravity settler outperformed the vortex (80.8% vs. 74.2% overall). The results show that both grit removal units had comparable overall removal efficiencies at the lower flow rate (38 ML/d or 10 MGD scale), 88.8% for the gravity settler and 87.5% for the vortex. Neither unit obtained the manufacturers stated efficiencies, but this was attributed to the possibility that the specific gravity of the raw influent grit was lower than 2.65. Table 1 Pilot Unit Removal Efficiencies
At the 57 ML/d (15 MGD) scale flow rate, the gravity settler outperformed the vortex for each range of grit particle size (>297μm, 210μm, and 105μm). At the higher flow rate of 57 ML/d (15 MGD) the vortex unit failed to remove grit particles in the 105 to 148 µm range. The gravity settler unit removed 32% of the grit particles in the 105 to 148 µm range. There were also negligible differences for the individual particle size ranges at the 38 ML/d (10 MGD) scale flow rate. The full scale cost for design and construction of each grit removal technology was estimated to be very similar. Two gravity settler units were priced at approximately $1.3 Million versus a $1.2 Million for two vortex units. The overall foot print for both technologies was again comparable and negligible, thus equipment selection would be based on performance data.
CONCLUSION Based on the pilot testing results, the municipality is proceeding with full scale designs for the gravity settling unit.
REFERENCES Willmott, N.K., & J+A Technical Department (Eds.). (2001). Jones+Attwood Technical Handbook. Issue 10. Page 1.
Grit vs. Grit
