HESCO ProactiView on Grit Capture, Transport and Removal
Key Words: Grit, Wastewater, Screening, Removal, Collection System, Digester
Grit is a
problem for all publicly owned treatment works (POTWs). It is small dense material, such
as broken glass, metal shavings and particles, silt, pebbles, &
aggregate that ends being captured in the collection system and
transported to the POTW. It
ranges in size from very fine (approximately 50 mesh or 300 microns) to
fairly coarse.
If these particles are not removed they abrade
pumps and other mechanical devices, causing undue wear and increased
maintenance costs.
In addition,
grit has the tendency to settle and collect in corners and bends, reducing
flow capacity and ultimately clogging pipes and channels. It also collects in sludge
treatment processes such as digesters, which reduces the active volume and
treatment capacity of the digester.
Removing grit from a digester and
repairing equipment damaged or worn by grit, is a difficult and costly
burden for maintenance personnel and their budgets.
This issue of the ProactiView
digs in to the Grit, and invites you to attend a 4-hour
comprehensive "Grit School".
Contents
The Solution
The Challenge
Unit Processes
Quantity of Grit Varies with Several Factors
Confirming Proper System Operation
Current Trends and Issues
Follow up with HESCO to Attend "Grit
School"
The
Solution
The simple solution to
these problems is to remove the grit before it enters these
processes.
The
Challenge
This grit removal process then becomes the challenge for the
Designer and ultimately the Plant Operator.
People involved in this process must understand the answers to these questions:
1) What is grit?
2) What are the purpose & objectives of grit removal?
3) What unit operations make up the grit removal system?
4) What variables affect the quantity and characteristics of the grit?
5) How do velocities affect grit transport and removal?
6) How does grit flow in channels, pipes & flumes.
7) Where should grit removal process be located?
The basic Grit Removal System consists of several unit processes:
- Grit Capture: via one of several available methods including vortex systems, aerated channels and velocity controlled channels.
- Grit Transport: moving the grit from the point of capture to the washing & disposal location
- Grit Washing: removing organics from the grit
- Grit Dewatering & Discharge: removing water from the grit and discharging the cleaned grit to the disposal hopper.
Grit
Capture: How do
you select what type of grit removal system to
use?
There are several general categories of grit removal systems based on the mechanism of grit capture. Each has their distinct advantages and disadvantages which must be taken into account when designing a new facility or planning upgrades or improvements to an existing facility.
Aerated Systems High energy costs due to aeration, simple channel layout.
Gravity Based Settling Systems Low operating costs, large footprint, narrow operating range.
Velocity Based Systems Simple channel layout, difficult controls or narrow operating range.
Vortex Based Systems Low energy costs. Small footprint. Strict channel configuration requirements.
Grit
Transport: Once captured, what is the best way to transport
Grit to the discharge location?
Often it is mechanically pumped
from a submerged collection chamber.
Sometimes airlift style pumps are used to transport it from this
chamber. Other situations
lend themselves to using a partially submerged screw conveyor, or a chain
& flight type scraper.
Choosing the right transport method requires understanding the
advantages & disadvantages of each, and the challenges posed by the
layout of the facility.
One of the most common problems associated with transporting grit via pumping stems from the piping configuration. Careful attention to the piping layout and distances can avoid these problems or eliminate existing ones.
Grit Washing:
Why should you wash the Grit before storage or
disposal?
Grit carries
and is often coated with organics.
It is preferable to keep the organics in the influent stream so
they can be treated by the biological processes at the plant. Too much organics in the grit can
cause odor problems for the plant and surrounding neighbors, as well as
lead to problems finding off-site disposal options.
This has led to grit washing systems which aggressively clean the grit, removing the organics and placing them back into the influent stream to the plant.
Grit Dewatering and
Discharge: How is the water
removed?
Once the
grit is cleaned, it should be dewatered so disposal costs are not paying
for hauling & disposing water.
Commonly this is done by screw conveyor, but is also achieved to
some degree in the collection hopper.
Quantity of Grit Varies with Several
Factors
What
influences the quantity of grit that a plant can
receive?
- Type of collection system - combined versus separated.
- Age of collection system deteriorating pipes and Inflow & Infiltration can contribute to the grit load.
- Make up of the collection system - heavy industrial versus food services, versus domestic.
- Flow rates & velocities in the collection system. Grit starts to move only when velocities become greater than approximately 1.0 to 1.5 ft/sec. Therefore grit often comes in loads with the peak flow rates as opposed to continuously throughout the day.
How do you
know if your grit system is performing properly?
Just looking into the collection
hopper may seem like the obvious answer, but that will not tell you how
fine of grit it is capturing.
Fine grit can be more trouble than the coarse grit because it can
work its way into seals and other tight tolerance components and really
increase the wear. This is a
chronic grit problem, whereas coarser grit can cause an acute problem like
jamming a piece of equipment, or clogging a pipe. Coarse grit can also cause chronic
problems when it builds up in channels or tanks reduces their volume and
the process treatment efficiency or capacity.
Testing the grit removal performance of the grit system can be done for two reasons:
1) On a new system when it is installed to ensure you received what you specified and purchased. This falls under performance certification.
2) On an existing system to determine if the existing grit removal system is in need of attention, or is the possible cause of excessive wear on downstream equipment. This falls under system diagnostics.
There is an industry trend towards finer and finer screening. Advanced treatment processes such
as Membrane Bio-Reactor (MBRs) are one of many factors influencing this
trend.
As the screening equipment becomes finer, larger sized grit can be captured by the screen, which may cause problems for this equipment.
This brings up the question of where to locate the grit removal equipment. The conventional location for grit removal process is after screening. The screening removes trash such as rags, straws, paper, and large objects that can foul pumps & valves and other mechanical equipment, including grit removal unit processes.
With some treatment facilities, it may become common to see a coarse screen followed by grit removal, followed by a very fine screen.
Follow up with
HESCO to Attend "Grit School"
This article
has raised many of the pertinent questions related to the Grit Removal
Process, and introduced some of the challenges associated with designing
and running this process. Further information on Grit
Removal can be found at our website.
To become an expert on this
process, and the variety of solutions available on the market, as well as
learn how to troubleshoot and improve existing systems, and properly
layout a new system, HESCO invites you to join us for a
FREE: Grit School
Seminar.
This
"
If you are:
- POTW personnel dealing with this process and/or the problems caused by grit.
- An experienced Engineer involved in designing a new plant or a plant upgrade with this process.
- Or a young professional interested in expanding your education & experience in treatment processes.
Please indicate your interest by clicking HERE . Please include your name, contact information and location.
We will coordinate with the respondents to establish a convenient date and location.
There are two immediate ways that you can contact us:
1. Call Kevin Livingston or Glenn Hummel at 586-978-7200 and mention that you are following up on the ProactiView on Grit Capture, Transport and Removal. We will be happy to discuss with you the details of your specific application or get you registered for Grit School.
2. Email Kevin Livingston or Glenn Hummel directly. Be sure to include your name, contact information, and brief description of your situation, needs and questions.