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Great article - Dan H.

DanHolohan Member, Moderator, Administrator Posts: 16,487
Pat O'Neil did a terrific job on a subject most folks never give much thought. Thanks, Pat!

Retired and loving it.


  • Now, this guy really knows his...

    sewage treament plants. Great job Patrick.

    Here's a question for you. Here in "drought ridden" Denver, I've always wondered what the minimum influent flows in GPM would be, and if that was any kind of an indicator of leaks from domestic appliances etc.

    I think the majority of the water cooled refrigeration equipment has been replaced (illegal for new installations), so what ever the minimal flow would be should be indicative of water conservation potentials if all leaks were fixed, correct?

    Also, here in Denver the sludge is recycled as a fertilizer (Metro-Gro) and is used on the parks for lawn fertilizer. Rumor has it that they ae considering selling it to the general public. What's your thoughts on that?

    Too many stinkin' questions?



    PS, I've toured our sewage treatment plant twice now. Cool stuff.

    PSS You forgot to mention the methane produced and how it can be used to power reciprocating engines to generate electricity and hot water.

  • PJO
    PJO Member Posts: 140
    ME and Dan,

    Thanks for the thoughts. I mean what I said about trying to "give back"...though it couldn't possibly equal what I 've learned here.

    Never too many questions Mark...and it's great to answer them if I may.

    1) Minimum influent flows...while every plant is different, most are designed on two things... Base flow, and maximum flow. I will only talk about sanitary flow only first...then combined flow (storm and sanitary).

    Picture a flattened bell-shaped curve that happens twice a day and you have the basis for your everyday flow in a typical plant. The first curve rises up from it's base flow at about 5:00 a.m. or so and keeps slowly increasing until it peaks between 8:00 and 10:00. It may drop a bit if the plant is smaller and has no industrial/commercial connections as a large percentage. The curve will flatten out somewhat in the middle of the day, then slightly increase again in the early evening until 10:00 or 11:00 p.m. If there is a large commercial connection, you have one long (flatter) bell shape for the entire day. The differences are typically about 2 to 1 or 3 to 1.

    Exceptions to this rule...plants that are tied directly to large commercial/industrial facilities, or have the majority of it's flow from them. Also, very large plants have a much steadier flow. I went through the Northeast Phila. plant some years ago, and they only vary maybe 50% all day. This is due to the length of piping in the system (flow may take 16 hours to get to the plant), as well as industries going 24 hours a day.

    For combined flow systems, the largest flow is nearly always the rain event. In Scranton's case, the "dry" flow was rarely above a rate of 14 MGD (remember, it's not from Miller!) at the plant, but the combined flow could top 60 MGD, and have twice that by-passing in the system. Most large, older plants are similar.

    Now, for the answer to your question :-)
    We went through a drought in the summer of 2001 in Scranton...nothing like you are in now, but this may help. The flow for an entire day would be about 6 or 7 MGD instead of 10 or 11...again, this is just sanitary. Yes, it may indicate some leakage but I agree that it would be a sign of conservation potential.

    I'm sure water is not allowed for various stupidly wasteful things like lawn sprinklers and the like, but you also must remember that a lot of the wasted water use does not ever get to the plant in the first place. Washing cars, driveways, etc. or lawns/gardens, etc. does not reflect in the sewage plant, but is very hard on the water plant. Not to change the subject, but in a drought period the water plants have it rough...usage is up and the quality of the "raw" water is down.

    The worry at sewage plants in a drought is the amount of food for the biomass...low flow and an average organic load means you have to reduce the bug population to avoid a starve-off. When you reduce this population it leaves you more vulnerable to a toxic shock, a sudden high flow (spike) from a big thunderstorm that passes though, etc. It's not a simple matter of opening a valve and the bugs just go away, and when you need to build them up again it takes some time.

    Question #2) I agree with selling or giving away the stabilized sludge to residents if the following conditions are met.
    1) There is ample warning that this is not to EVER be used to fertilize any garden that has the potential for food ingestion...this includes herb gardens, berries, etc.

    2) There is ample periods of sludge stabilization. This is more complicated, but basically you have a minimum amount of time for the bugs to die-off in the heat of composting, you must add a pH adjustment chemical (lime), and the facility should have ANY and ALL information about the compost always available to the public.

    Scranton used to give it's compost away to anyone, and the feedback was generally very good. Philadelphia used to sell it, but now gives it away like your plant, or simply puts piles of it at city owned lots and it "disappears" on the beds of landscaping trucks. This is, IMHO, controversial at best...and bad anything below that because it is not regulated. I may get in trouble for that statement but I saw it with my own eyes a few years ago.

    The reason in general for the regulation is heavy metals. Every plant of a certain size gets them in the influent, and guess where it ends up? Scranton, like most plants, has quarterly testing done to the sludge for all kinds of stuff, and it is normal to have a certain percentage of metals...it's better than going to the river. This is the main reason IMHO for my answer to the residents taking the sludge.

    #3) I did not mention those uses due to the length of the document. Yes, ME you are correct about the use of methane. Scranton simply stores there sludge in an aerated tank before processing, but most large plants use digesters. I was contemplating the mention of this topic, but I chose to leave it out...now I regret it. I also left out plant maintenance, use of heat exchangers/water to water heat pumps to extract the heat from the flow (mentioned it in Scranton for the effluent flow when I was there and I got a crazy look from the boss) and the basic reactions that happen through the plant.

    As far as re-use of methane goes, it works very well in most situations. Anaerobic (no air) bacteria slowly break down the waste in very large tanks (digesters), and it takes between 2 and 8 weeks normally. Imagine how much land you need just for that...N.E. Philly had about 16 tanks that where probably 400,000 gallons each. As the sludge is broken down, it gives off methane...you better get rid of it from the tank or you have big problems - including explosions. When you see a flame on top of a tank at a WWTP, it's burning off the methane that's not being used elsewhere for heat. If you don't burn it off...PHEW!

    Wow did this get long...sorry, it's the heritage :-)

    Take Care, PJO

  • billygoat22
    billygoat22 Member Posts: 124

    Sludge is called biosolids here in Va. Several companies from northern states are importing the stuff and "giving" it to farmers as fertilizer, spreading it, too. This is happening all over the state because stae laws were changed to prevent local governments from stopping its usage, even when overwhelming citizen petitions were presented. The contractors threaten to sue local government if they try to pass restrictions. The sludge comes from states who have outlawed dumping it as fertilizer. They just started in Appomattox, Va. a few months ago, the farmer down the street from me had it dumped on his fields. People fear disease and pollution from it and I have to agree, you can't get all the bad stuff out, especially if your drinkung water is only 30' below the soil and 1/4 mi from one of those fields. Hate to sound confrontational about this.
    I'm glad Scranton is spending their money to landfill it, like our local governments do, Thank You. I wonder if Scranton has a policy like our town , not to export our problems to other towns?
  • PJO (@ home)
    PJO (@ home) Member Posts: 1
    Sludge recycling...

    I'm not sure of Scranton's policy on that, and I also don't disagree with the land use of this...IMHO it is better than landfilling IF it's done right.

    Of course, everyone's opinion of right is different :-)

    I personally like to see land application if; 1) The land it's going on is not a grower of food or products that end up being ingested down the line (like corn for cows, etc.) 2) The biosolids are documented and tested well...this includes the tests mentioned earlier, plus the transportation manifests and specific guidelines on the application.

    As far as the groundwater, it is a worry - especially with shallow wells. The soil layers do an excellent job of "purifying" the water, but it needs time and layers to do it correctly. The nightmare is when there's a heavy rain the night of an application that was already too heavy...right to the local (surface) waterway it goes.

    There's so much of this stuff that it has to be recycled, and it's a natural fit for fertilizer due to the high percentage of nitrogen-based constituents from the treatment process. When done right, it's a win-win...I just worry if it isn't. The heavy metals are also a worry, but many biosolids also have very low numbers (sometimes none) of these. I believe it is for public record once the testing is reported to the EPA and state authorities, so with some patience and work you could find out.

    Hope this helps. Have a great Saturday Wallies!

    Take Care, PJO
  • billygoat22
    billygoat22 Member Posts: 124

    PJO-all the points you would desire for land application are not in use in Va.- it is spread on farmland with limited time restriction untill products can be produced on that land. Appomattox wanted injection which would prevent runoff and would allow wetter sludge to be applied. They are spreading the class "A",I think, type which has more testing. The biosolids cos are threatening/suing if counties demand better testing, initial tests on land for a baseline comparison, and injection, and especially if they try to restrict its usage.
    I'm not worried about the biohazard some think exists or the smell that sometimes occurs. Its not the biosolids but what imdustries let down the drain that I'm worried about, you can't possibly test for 1000s of chemicals and the not knowing is the dicey part. How can it be safe if noone knows whats in it? Incidentally, they say commercial fertilizers have heavy metal content similar to sludge standards.
This discussion has been closed.