Grantee Research Project Results
Final Report: High Rate Sedimentation Tank for Hydraulic H2O Treatment
EPA Grant Number: SU836772Title: High Rate Sedimentation Tank for Hydraulic H2O Treatment
Investigators: Weber-Shirk, Monroe
Institution: Cornell University
EPA Project Officer: Page, Angela
Phase: I
Project Period: September 1, 2016 through August 31, 2017
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2016) RFA Text | Recipients Lists
Research Category: P3 Challenge Area - Safe and Sustainable Water Resources , Sustainable and Healthy Communities , P3 Awards
Objective:
Sedimentation is a critical process for water treatment plants. It is the process by which coagulated minerals, dirt, clay, and other particles are removed from the water via gravitational settling. The particles settle into a "floc" — a fluidized bed of suspended solids colliding in a bottom zone of the tank. The particles are initially light and small, but as coagulant dosage persists and particles continue to collide, the particles clump together into heavier floc that will settle into the basin of the recirculator. This process permits clearer water to continue up the plate settler, resulting in a lower effluent NTU (Nephelometric Turbidity Unit, a measure of clarity).
AguaClara's sedimentation tank design includes inclined parallel plates called plate settlers. The purpose of these plates is to catch small particles and return them to the floc blanket developing in the base of the tank. In the AguaClara lab, a sedimentation tank and its respective plate settlers are simulated by tubing. The tube that simulates a pathway of fluid in the tank is referred to as the "recirculator" by the High Rate Sedimentation (HRS) team. The slanted tube that simulates a plate settler is designated as the "tube settler." The plate settlers increase the amount of horizontal area for the flocs to settle out. Due to their sticky nature, these flocs aggregate; growing in size as they slide down the plate settler and back into the basin.
Summary/Accomplishments (Outputs/Outcomes):
AguaClara designed a vertical sedimentation tank, which has the water ow from the bottom of the tank to the top. The ow velocity that maintains the oc blanket is known as over ow rate or up ow velocity. Flow rate (Q), up ow velocity (V), and tank surface area (A) are related to the continuity equation:
Q = V A
The AguaClara HRS team hopes to design a tank that will yield an effluent of 0.3 NTU or lower with maintaining high upflow velocity. While the World Health Organization has a standard of at most 1 NTU for drinking water, the EPA standard is 0.3 NTU. AguaClara currently achieves this with an upflow velocity of 1 mm/s, but not at upflows of 3 mm/s or greater.
The Fall 2017 team has reduced the size of the sedimentation basin by half and transitioned to a 1 inch tube diameter. Although geometry is minimal and upflow velocity was high, the Fall 2017 team plans to achieve NTUs below 0.3 through floc size selection, a feat manageable through capture velocity manipulation. The reason for this is to find alternatives to the complex geometry indicative of the trapezoidal design fabricated by the Spring 2017 team.
The results of past experimentations indicated that modifying the recirculation zone of the sedimentation tank enabled increased upflow velocity and flow rate, without compromising the water quality. A high rate sedimentation tank can be designed that could have the capability of producing the same quality of water, but in a more compact size. Because sedimentation is the slowest unit process of the treatment process, using higher upflow velocities to reduce residence time and treat more water at once is essential. This will save time, money, space, and materials required to construct this section of a standard AguaClara plant.
Conclusions:
This semester's work raises more questions than it answers. Although the Fall 2017 team worked extensively with apparatus geometry, specifically weir location and number of weirs, the direction of experimentation shifts when the team observes a consistent drop in performance over time in each experiment. The team investigates the floc blanket degradation phenomenon and attempts to pinpoint its cause. Despite eliminating shear pulses from the list of possible causes, it is found that the floc blanket performs worse with a steady supply of water versus an intermittent, pulsing flow rate.
However, removing pulsing shear forces from the equation does not solve the increase in headloss within the flocculator. This supports the hypothesis that there is floc accumulation on the inside of the flocculator tubing that is proving detrimental to long term performance of the sedimentation process. Further experimentation is needed in order to confirm these suspicions.
From these experiments, it is abundantly clear that before experiments at high upflow velocities can continue, this problem of floc production and quality must be solved.
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.