designing WWTP for tourists place - different loads

[MirJev]

Need your help guys again!
I am supposed to prepare preliminary design for treating municipal wastewater for two neighboring villages. The issue is that there is a large difference in population throughout the year, due to the tourist season (summer). The first village has 350 permanent residents. During the summer there is additional 5000 people (total 5500 PE). The second village has 320 permanent residents, and additional 950 people in summer (total 1500 PE). The Investors' wish is to have compact, easy to operate and maintain, underground WWTP or WWTPs. As far as I see it, there are couple of possibilities:

1) One centralized WWTP, 7000 population equivalent (PE).
2) Two separate WWTPs, for each village, first 5500 PE, second 1500 PE.
3) 4 separate WWTPs, two for permanent residents in both villages (350 PE and 320 PE), that would work throughout the year, and two for tourists that would work only in summer (5000 PE and 950 PE).

Question: referring to possibility 3: Is it possible for a WWTP to be in some kind of a stand-by regime when tourists are not present? How easy is it to establish biological process again? I guess that activated sludge from other WWTPs could be used to start the process.
As usual, I would be grateful for your suggestions and advices.
Thank you

 

[MirJev]

I would like to add one more possibility:
4) One centralized WWTP, for permanent residents of both villages, designed for 670 PE, but with a large equalization tank that would be used mostly in summer, to flatten the peak hydraulic and organic loading

 

[ashtree]

MirJev

This is a really tough ask and your investors are asking for some quite difficult outcomes.
I am going to list some assumptions for a start.
1) All the wastewater regardless of the time of the year has to be treated to a similar high standard to make it suitable for some sort of reuse or environmental discharge.
2) All normal facilities such as mains power etc are available
3) Your summer peak is about 3 months, which means you treat wastewater for 670 people 9 months of the year and 7000people for 3 months of the year.
4) Your regulatory body eg: EPA does not impose any sizing or capacity requirements upon you.


First of all to answer your Q4. I would suggest some equalisation capacity will be needed but you wont be able to build a tank big enough to flatten this peak. Likewise wastewater will turn septic if held for too long and this will cause odour issues and be problematic for treatment down the track. Anything more than a few hours will be a problem unless using oxygen, chlorine or ozone injection up front somewhere.

The question of one or two wastewater plants is going to be largely an economic one. I would think that one plant will have more success in achieving your goals and might give you more capability to handle the flow regime. One plant will be easier to operate and maintain then two. One larger plant will probably be cheaper to build because of economies of scale. But how much will it cost to pump wastewater from one village to the next in both capital and operation. Is the terrain favorable for pumping, are there sites available for a larger plant etc. Ideally you would locate the plant near the largest village so you have to pump the least but maybe the terrain will dictate something else.

Notwithstanding the previous paragraph i don't think four plants will be an option. First of all you wont have enough activated sludge in the two all year plants to adequately seed the two summer plants such that they can very quickly get going. It will probably take them a month to get established and three months to be working well assuming everything goes well. At that time you have to start pulling back to shut down the two plants and you will have a lot of sludge to dispose of in a short time. I have seen established plants come back from total sludge washouts due to severe flooding producing high quality effluent in about 7 days but this is with very experienced operations staff using some seed sludge from aerobic digestors to get them going again. In one instance though the plant then took a very long time to optimise probably due to some shortcomings in the bug population.

In technical terms you are going to have to deal with a 10: 1 load turn down.

Without knowing all the specific details this would be roughly my design approach assuming a single large plant. You have climate on your side however as it will be easier to treat the largest volume in summer as compared to what happens in ski villages.

Inlet works: Three parallel trains. use one all year and bring on two extras for three months. You might accept different or lesser technology in your seasonal equipment.

Flow equalisation basin: This would need to be big enough so that during three months of the year you can feed at a consistent flow rate all day. This will have to be sized to cover the peaks and troughs. The magnitude of those peaks and troughs will be depenednt on the size and shape of the villages, whether flow to the plant is gravity, pumped or mixed, the habits of the residents etc. During the off season the equalisation basin might be taken out of service.

Bioreactors: Lets assume some form of extended aeration is used and probably the industry will lean towards an intermittent type of process. You would probably want 2-3 tanks and you might take one or two off line during the off season but they would need to be sized and equipped to take the peak flow during holiday season. If the equalisation basin is sized correctly considerable money can be saved here.
In the peak time you might operate at at a lower SRT maybe 8-10 days but in the off season a lot longer 20-40 days. Depending on your tank arrangements summer time HDT might be 12 hours but this may go out to 24 or 36 hours in the off season. Blower and pumping capacity will need to made up through the use of multiple units with plenty of turn down through the use of VFDs and a really flexible control system.
Sludge management will have to be carefully thought through but it maybe that you run any dewatering equipment hard and long during the peak times and have much shorter use during the off season.

As for all this being compact underground and simple and easy to operate and maintain. Well anything is possible if you have enough money but i think "the investors" will need a reality check soon. 7000 EP even if being really optimistis will probably have 300,000USG of bioreactor volume.

Some one will probably jump in here and say that it can be done with an MBR much cheaper easier with less volume etc. Maybe that will be the case but the general principles i have outlined here will be the same even for an MBR.



Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[MirJev]

First of all I want to thank you for your detailed answer.
I thought of MBRs, but I think they are quite difficult for maintaining: backwashing, chemical washing, taking out the mambranes for detailed chemical washing and after all of that, replacement is inevitable.
As far as I understand it, your design approach is a combination of my solutions #4 and #3. You stated that it would be difficult to restore the biological process quick enough, but your design implies using multiple biological tanks, with possibility to take one or two off line during the off season. Than your design would have the same problem, wouldn't it?

Regarding the SRT: Due to the fact that local regulations don't impose any restrictions regarding the nitrogen, I assumed SRT of 4-5 days would be enough for the summer, and 10 days for winter. I only need carbon removal in my process. If you have any advices or lectures regarding the SRT, I would be grateful if you could share it, because determining SRT is mostly a matter of experience, and I don't have a lot of it

So, I can now narrow down my solutions to #3 and #4. For #4, I would probably add some simple mechanical aeration in equalisation tank, in order to prevent septic conditions. Regarding the #3, I will ask the potential equipment provider (Nijhuis) if they have some data on how much time is it needed for biological process to be established.

 

[ashtree]

A couple of things in response.

Personally i don't think MBRs are the right solution either, but i am sure there may be people out there who think they could be.
Your comments about the multiple tanks are correct, however it is much easier to bring on another tank than to take a similar quantity of seed sludge to another plant some distance away and then get that plant established quickly. No matter what method is employed if the change from off season to on season is very quick it will be difficult to transition quickly and to get the plant established for the much larger load. However it will be much easier just to bring on another tank, because basically you divide the flows and the available sludge between the tanks online.

If nitrogen removal is not required and assuming you don't want to do nitrogen removal than you need to run at much lower SRTs during the summer. But this no nitrogen requirement will make an easier design and reduce tankage requirements. Without knowing what your summer temperatures are i would suggest 1 to 2 days SRT will be the likely range because nitrification should be avoided. However i think it is very likely that during off season unless tankage volume is dramatically reduced nitrification and longer SRTs are almost inevitable. Therefore there will be a requirement to denitrify and this has to be considered in the design.

Whilst i would think that multiple tanks is probably still required swing zones equipped with suitable recycles and mixers might be used advantageously. This would involve having parts of the aeration tank divided off and used as an anoxic zone during the off season but aerated in the summer months when loads are higher but denitrification is not required. An intermittent process might be divided by time/sequence as well as physically.

Equalisation basin will almost certainly require some form of mixing.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[MirJev]

I am currently doing some calculations for equalisation basin. I am doing calculations for the WWTP for 670 PE (134 m3/day), with an equalisation tank that would manage the peak flow for 7000 PE (1400 m3/day) (VERSION #4). I came to a solution that 400 m3 equalisation tank would be enough? What is your opinion?
I can't use the M&E way of determining the volume for equalisation tank by plotting hourly flowrates, because I don't have the flowrates. There is no sewerage network built.
My older colleague says he determines the volume by simply multiplying the daily flow with 0.5 - 0.7. I think that the volume of 980 m3 (1400 x 0.7) is way too big.

My way:
Based on a calculated aeration tank volume (40 m3), I determined hydraulic detention time for off-season flow (134 m3/day), which is 7 hours. Than I reversed the calculation for the peak flow, and got the equalisation tank volume:
7hours / 24 hours/days x 1400 m3/day = 400 m3

Does this make sense?

 

[MirJev]

The more I look at it, the more I am aware that it doesn't make sense. I guess that, considering the fact that bioreactor is designed for 134 m3/day, the equalisation tank of 400 m3 is far too small. Furthermore, I would say that it is practically impossible to do the version #4, as you Ashtree already said in your first post.
Going back to the drawing board
I guess I will have to go with bigger plant that would work under lower loading most of the year, because the difference in season and off-season flow is just too big.

 

[bimr]

You need to know the discharge requirements in order to develop a plan. Are the removal of nutrients required?

RBC facilities have been used for resorts where the loading varies. It is possible to operate multiple RBC at peak loadings and a single RBC at off peak loadings.

 

[MirJev]

@bimr
Nutrient removal is not required.
And what is the case with RBC and required time for starting and establishing the process? I guess that there is no possibility, like with suspended biological processes, to take activated sludge from some other plant?

 

[ashtree]

One centralised WWTP will be the best way to go if the constraints like terrain, land availability , pumping distances etc are not limiting.
As bimr suggest RBC have been used in these circumstances and would do the job fine but start up times may be just as much a problem as activated sludge. In any event the broad principles around sizing and turn down would still apply.
If you went with a single WWTP i would suggest that the equalisation basin would be off line during the off peak season and only used during the peak times.
Without knowing the specifics of the site you might have a daily flow peak of 3 times average dry weather flow but you might size your equalisation basin to limit this to about 1.5. This will save you money in Capex in tank and equipment size reductions and probably Opex in that you might be able to optimise closer to maximum efficiency. Maintenmance costs may be lower as well. The reduced size will also make things easier during the off season. You will still have more capacity then you need and flow equalisation wont be required.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[MirJev]

@Ashtree
thank you for the details on equalisation tank.

Another question: is it possible to maintain MLSS of 2500 g/m3, when SRT is 3 days? I guess it is not possible. But if I assume lower MLSS for my system (perhaps 1000-1200 g/m3), I need 2 times larger aeration tank.

 

[ashtree]

Whilst you can make assumptions and design for a certain MLSS, the actual MLSS depends on a number of factors including the actual strength of the wastewater, the sludge yield from that waste water, the SRT, temperature and the volume of the aeration tank.
Given that you need to allow for a turn down of close to 10: 1 i would try to design the aeration basin as small as reasonably possible in line with achieveing the treatment objectives. This will mean operating with and designing for a fairly high MLSS (4000mg/l perhaps) during the peak times.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[MirJev]

Yes, but MLSS of 4000 mg/l is typical for extended aeration process, and I am assuming SRT=3 days, in order to avoid nitrification. That is why I am asking if it is possible to run MLSS of 2500 mg/l with SRT=3 days.

 

[ashtree]

Yes. But i am not certain you understand the relationship correctly. For a given plant with certain tank volumes and a particular wastewater there will be a certain relationship between SRT and MLSS.
But to simply link 2500 MLSS and SRT of 3 days is wrong just as much to assume that an MLSS of 4000 must have an SRT long enough to nitrify, because there are many design factors to consider.
You are correct that often plants that nitrify will have often have higher MLSS but this is not necessarily automatic.
To clarify i will share two extreme examples from my past.

Plant No1- 700,000 USGPD conventional activated sludge plant not designed for nitrification. It was only recieving about 700,000 USGPD but had been designed for a BOD of about 150mg/l but was getting well over 250mg/l. At about 2 day SRT it maintained 4-4500mg MLSS even though it was intended to have only about 3000 MLSS. However there was insufficent sludge handling capability to operate at 3000 MLSS and the SRT would have been really short. HDT was under 12 hours. The plant was overloaded from a biological perspective so that the amount of sludge carried was very high. The plant in summertime would sometimes show ammonia reductions so was nitrifying somewhat, but only occassionally did this cause problems.

Plant 2 - 120,000USGPD extended aeration plant designed for BNR. Plant was lightly loaded with less than 30% load. The plant had quite large process volumes so at 30% load had something close to 4 days HDT and was operating at about 1000mg/l MLSS and close to 40 day SRT despite being designed for about 24 hour HDT and 2500MLSS. The plant removed nitrogen very well despite being very lightly loaded.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

If you have no nutrient removal requirements, RBC systems are the best option.

Startup of an RBC system will only take a few days versus months for an activated sludge system to stabilize.

Operation of the system is easier for the plant operator.

 

[ashtree]

Assuming that RBCs can achieve the licence standards RBCs will be fine.
But as i said in earlier posts the general principles on sizing for turn down and equalisation basins will still apply.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"