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WaterOperator.org Blog

Articles in support of small community water and wastewater operators.


Jennifer Wilson
Jennifer Wilson
Jennifer Wilson's Blog

Screens: An Important First Step in a Wastewater Treatment Plant

Screens: An Important First Step in a Wastewater Treatment Plant

 

 

By Phil Vella

No matter what size wastewater treatment plant you have, screening equipment at the headworks is a necessary requirement. Screens or pretreatment devices are designed to remove or reduce large solids like wood, cloth, paper and plastics from the waste stream. This not only allows downstream treatment process to be more efficient but also protects the equipment such as pumps.

Several different types of equipment can be used to meet these objectives and there is no one-size-fits-all solution that can be applied to every headworks situation. Some of the limitations of small systems are low flows, space and financial considerations. The following will focus on those options most likely to fit into a small wastewater treatment plant. 

As with most equipment at a plant, screens come in a variety of sizes, capacity, automation and cost. In general screens may be classified as coarse, fine and micro and are based on the size of the screening openings. The discussion here will focus on course screen technology with openings 6 to 36 mm (0.25 to 1.5 in.).
 

Manual Bar Screens

With the limitations of small systems, a manual bar screen may be a great option. These screens have vertical bars approximately 1 to 2 inches apart to catch the incoming debris. Although very basic, they do provide a good level of protection for the plant. An example is shown in Figure 1. However, as with most basic equipment, there are limitations.


Source: Islamic University of Gaza.

The most obvious limitation is that this is a manual operation and requires dedicated manpower and can be a burden to small systems. This is especially true during high flow events such as storms that may require more frequent raking and may also create more of a safety hazard for the operator.

Automatic Bar Screens

To limit the labor involved with manual bar screens, there are several automated options available. These can be classified into different group types. Chain Driven Screens, Catenary Screens, Reciprocating Rakes, Continuous Belt Screen and many variations of them. A summary of different types of screens with their advantages and disadvantages is given in Table 1. These options also commonly use vertical bars to capture the solids and remove them with an automated raking system. Since these are automated, the cost and other operational costs must be considered. In addition, these systems are larger than the manual screen so adding this to a facility may require civil engineering to modify the influent channel or the headworks building if placed there. You may have reduced the labor cost but have increased the capital and infrastructure expense.

Although course screens can remove large material at the head works, disposal of this material becomes and added cost and requires operation and maintenance.  In addition, the wet screenings collected are smelly that can attract vermin and result in odor complaints from the community. 

TYPE OF SCREEN

ADVANTAGES

DISADVANTAGES

CHAIN OR CABLE DRIVEN SCREENS

Design in the market for many years
Simple channel construction
High screenings loading rate
Insensitive to Fat, Oil, and Grease (FOG)
Low headroom required

Submerged components subject to wear and tear

RECIPROCATING RAKE SCREENS

No critical submerged components
Widely used

Low screening loading rate
High overhead clearance, particularly at deep channels

CONTINUOUS SELF-CLEANING SCREENS

Medium to low headroom required
Allows a pivot design for servicing the unit above the channel

Several moving components
Components subject to wear and tear

ARC SCREENS

Simple design
Lower capital and operational cost
No drive parts under water Utilizes 100% of channel width

Limited to small to medium flow plants
Not suited for deep channels

CATENARY SCREENS

Simple to operate
Easy to maintain

Chains are very heavy and difficult to handle
Large footprint

Source: WEF, Manual of Practice 8, 2017

The following are examples of some of the screen options available to wastewater treatment plants.

Multi Rake Chain Driven Bar Screen (Automatic/Self Cleaning) Source 

Multi Rake Automatic System

 


 

Reciprocating (Single/Basket) Rakes | Source

Catenary Bar Screen | Source

Arc Screen | Source

In summary, there is no right equipment choice for all headworks screening circumstances. Individual factors such as flow rate, solids loading, cost and infrastructure modifications must be considered. Choosing the correct option is important not only in protecting downstream equipment but also for efficient and effective solids removal resulting in proper wastewater treatment. 

New Video Series: Compliance Conversations for Small Water Systems

New Video Series: Compliance Conversations for Small Water Systems

Every time we hop on a call or head to a conference, we hear from experts with a wealth of knowledge. From "Why didn't I think of that?" tips to fascinating case studies, we're bringing the juiciest tidbits to you with a new video series.

Compliance Conversations offers insights on operating and managing a small public water system from a range of contributors. Each episode will feature an interview with a water industry professional so you can learn from the comfort of your computer or favorite device.

The first four episodes feature Jeff Oxenford, of Oxenford Consulting and the Rural Community Assistance Partnership. In these episodes, Jeff breaks down the most common drinking water compliance issues:

We're already working on a new batch of episodes, so make sure to subscribe to the WaterOperator.org YouTube channel so you will be the first to know when new videos are uploaded!

Free Resources for Non-Community Water Systems - Recording on June 6, 2018

Free Resources for Non-Community Water Systems - Recording on June 6, 2018

This webinar, recorded on June 6, 2018, introduces our free, 2-hour online course that helps owners and operators of public water systems with a groundwater well better understand how to properly care for their water supply. The course curriculum includes the basic science of groundwater, well mechanics, and source water protection best practices. 

What We Can Learn from Flint

It’s not often that drinking water gets in-depth news coverage and front page headlines, but I think we’re all just sad that it happened this way. The story of Flint, Michigan’s drinking water crisis has unfolded over nearly two years, but the national media attention escalated rapidly in the past month.

I believe I speak for every one of our WaterOperator.org readers when I say this just hits too close to home. This is our industry, these are our friends and colleagues, and of course, the people of Flint are our neighbors in trusting that tap water will always deliver.

There’s no role for blame because we’ve all lost on this one. And when you go beyond the issues of oversight, social justice, and politics, there’s a story about the challenging decisions that operators, utility managers, and local government officials make day-to-day. These jobs have aways been hard, but we now have an opportunity to grow, change, and do better.

This could have happened anywhere, but it doesn’t have to happen in your community. Here’s what everyone can learn from Flint:

Unintended consequences are real.

The story of Flint highlights the critical balancing act required to serve drinking water that meets every standard. One change (large or small) can have cascading effects on the entire treatment train and distribution system, so decisions should not be made lightly. Appendix C (Guidance for Evaluating Impacts of Treatment Changes on Distribution Systems) and D (Tools for Evaluating Impacts of Treatment Changes on Lead and Copper Rule Compliance) within the Simultaneous Compliance Guidance Manual are solid, first-step references.

Don’t be afraid to ask for help.

State and federal agencies are made up of people who care about what they do. So not only is it their job to help systems make better decisions, they want to do the right thing. They also know others with additional technical expertise, including researchers and technical assistance providers, who can consult with you at no cost. Ask for assistance when planning changes or as soon as you know there is a problem. If you’re not sure whom to contact, here’s the list of primacy agency websites. You can also contact us (info@wateroperator.org) and we’ll find someone who can help.

Public health is the priority.

A water system’s ultimate job is not to meet compliance, but to provide safe drinking water and protect public health. Regulations are the baseline mechanism for getting there, but thinking holistically about what’s logical can prevent unintended consequences. There are certainly flaws in the Safe Drinking Water Act regulations, so the Water Supply Guidance (WSG) manual offers policy statements and clarifications on intent as a starting point.

Trust is easier to break than restore.

It is always better to act out of an abundance of caution and be wrong, than it is to do nothing out of fear. Early, active, and consistent public communication (even when the answers are still uncertain) will go far to maintain the public’s trust in the water system and the local government. We’ve compiled some of the best resources on risk communication requirements and best practices.

The situation in Flint is more than unfortunate, but we can all reduce the chance that it will happen again and be more prepared to react in any emergency situation. Our thoughts are with each and every one of you working beyond measure to make this right.

Winterizing for Consumers and Small Water Systems

Here in central Illinois, the days are getting shorter, the nights are getting colder, and the trees are starting to turn. For those of us living in colder climates, the time is coming for us to batten down the hatches and prepare for winter’s snow, ice, and cold. Recently, one such forward-thinking operator asked us for information on winterizing service lines. After a little searching, here’s some guidance we found for him and for anyone else preparing their system for winter cold.

Winterizing for Water Systems

For operators looking to prepare their systems for winter, the Preventive Maintenance Card File for Small Public Water Systems Using Ground Water (developed by the U.S. EPA and adapted by the Massachusetts DEP) provides month-by-month guidance on routine maintenance procedures that can help keep a system in top running condition. Search the document using the keyword “winter” to find relevant maintenance cards. The Indiana Section of the AWWA also has a winterizing checklist. See page 8 of this newsletter for their helpful tips and hints for water operators.

Consumer Information: Winterizing Plumbing and Thawing Frozen Pipes

Of course, operators are not the only ones facing the problem of inadequately winterized or frozen pipes. Consumers often need extra guidance in properly preparing their homes for cold weather, or in dealing with frozen pipes as they occur. Some resources for consumer information include:
  • RCAP’s ebulletin on winterizing for the utility and the customer
  • the Red Cross’s information page on winterizing pipes, and safely thawing pipes that have frozen,
  • this video by a real estate agent showing how to properly drain outside spigots for the winter,
  • and this video by a building contractor in Boulder, Colorado, which includes tips for turning off water to the house in the event of a burst pipe, ways of regulating temperatures so pipes don’t freeze in the first place, and advice on safely thawing pipes when they do freeze.

To see how other utilities have handled consumer information on winterizing pipes on their websites, see the Mohawk Valley Water Authority (for colder climates) and the Macon Water Authority (for climates with relatively mild winters, where the ground seldom freezes deeper than two inches). Though there may be contact information or policy information specific to these utilities on these pages, both provide thorough, accessible information to frequently asked consumer questions.

Are there other great winterizing resources that should be highlighted here? Tell us in the comments!