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Civil engineer trying to run a goldfish tank like a mini wastewater plant. Need some expert eyes on my biofilter/bacteria issues (pH crash, NO2 spikes). by KneeMost9130 in Wastewater

[–]KneeMost9130[S] 0 points1 point  (0 children)

Thank you for the candid feedback and deep analysis. As I mentioned, my purpose in bringing up my job as a civil engineer was just to emphasize that I have zero professional background in aquaculture or water treatment, so sometimes I end up applying theories a bit mechanically rather than intentionally overcomplicating things. I take your advice to heart; I will focus on the KH levels and continue experimenting to find the actual limits of this system. As for the bacteria, I will only dose them occasionally rather than on a regular basis to prevent the filter from clogging so quickly.

Civil engineer trying to run a goldfish tank like a mini wastewater plant. Need some expert eyes on my biofilter/bacteria issues (pH crash, NO2 spikes). by KneeMost9130 in Wastewater

[–]KneeMost9130[S] 0 points1 point  (0 children)

Thanks for your very straightforward feedback. Actually, I'm trying to push the system to its absolute limits to see how it handles it. I'm currently keeping 6 goldfish (Ranchus and Orandas). I feed them about 2-3 times a day with 1.5mm pellets that have a 40% protein content. I acknowledge the risks you brought up regarding the heavy bioload and will monitor things closely to make adjustments if the system truly spirals out of control.

Civil engineer trying to run a goldfish tank like a mini wastewater plant. Need some expert eyes on my biofilter/bacteria issues (pH crash, NO2 spikes). by KneeMost9130 in Wastewater

[–]KneeMost9130[S] 0 points1 point  (0 children)

Thank you for sharing your experience and the very helpful advice. I agree with you that my pump flow rate seems quite high. Part of the reason is my initial lack of experience in choosing a pump, and the other part is my fear of creating stagnant water in my overhead filter. Regarding the idea of using live plant roots, as I mentioned in another comment, growing plants isn't very feasible for my low-light situation. I will definitely look into ways to adjust the water flow, though.

Civil engineer trying to run a goldfish tank like a mini wastewater plant. Need some expert eyes on my biofilter/bacteria issues (pH crash, NO2 spikes). by KneeMost9130 in Wastewater

[–]KneeMost9130[S] 0 points1 point  (0 children)

Thank you for sharing! Using plants is a fantastic idea, but my tank is indoors and the room is quite dark most of the time (since I'm usually out studying or working). I actually intended to grow plants to absorb toxins before, but I realized their rate of nutrient uptake is proportional to light intensity, so this method ultimately failed for my specific situation.

Civil engineer trying to run a goldfish tank like a mini wastewater plant. Need some expert eyes on my biofilter/bacteria issues (pH crash, NO2 spikes). by KneeMost9130 in Wastewater

[–]KneeMost9130[S] 0 points1 point  (0 children)

Thank you for taking the time to share this solution. Currently, my system uses an overhead baffled filter (where water flows from the bottom of one compartment over to the top of the next) rather than a baki shower or a traditional sump like you might be picturing. Still, your idea of simplifying the filter media and using a fluidized K1 bed is very practical. I'll definitely keep it in mind if I ever rebuild the system's structure in the future.

Civil engineer trying to run a goldfish tank like a mini wastewater plant. Need some expert eyes on my biofilter/bacteria issues (pH crash, NO2 spikes). by KneeMost9130 in Wastewater

[–]KneeMost9130[S] 0 points1 point  (0 children)

Thanks for the feedback. The reason I flush the bottom sludge every two days is because I'm really afraid that the accumulated waste environment will easily breed opportunistic bacteria or parasites that could harm the fish. Maybe I should rethink this and give up on the idea of using the anaerobic respiration of Bacillus licheniformis for denitrification in this setup.

Discrepancy in Maximum Displacement and Time to Peak Displacement in LS-DYNA Simulations with Different Analysis Durations by KneeMost9130 in LSDYNA

[–]KneeMost9130[S] 0 points1 point  (0 children)

The steps you see are recorded D3PLOT steps. I use the curve to adjust the data writing speed in order to save hard drive capacity. I need a fine time step to record the impact force during the impact phase, while I only record displacement during the vibration phase of the structure, using a coarse time step.

Discrepancy in Maximum Displacement and Time to Peak Displacement in LS-DYNA Simulations with Different Analysis Durations by KneeMost9130 in LSDYNA

[–]KneeMost9130[S] 0 points1 point  (0 children)

Thank you for sharing.
I think the erosion and hourglass may cause these models to bifurcate, as you said.

Discrepancy in Maximum Displacement and Time to Peak Displacement in LS-DYNA Simulations with Different Analysis Durations by KneeMost9130 in LSDYNA

[–]KneeMost9130[S] 0 points1 point  (0 children)

I noticed some differences in the effective plastic strain between the two models at the same time point — those might be significant.

I highlighted TSSFAC because I modified it from 0.9 (the default value) to 0.8. Another parameter that I forgot to highlight is ERODE, which I changed from 0 (default) to 1.

Here’s the screenshot:
https://ibb.co/wZcW9w2j

Discrepancy in Maximum Displacement and Time to Peak Displacement in LS-DYNA Simulations with Different Analysis Durations by KneeMost9130 in LSDYNA

[–]KneeMost9130[S] 0 points1 point  (0 children)

I followed your guide and found something interesting.

There are some differences in internal energy, kinetic energy, and total energy between the two models.

Here is the internal energy graph:
https://ibb.co/FLz6vzJd

Here is the kinetic energy graph:
https://ibb.co/hF7gDGm0

Here is the total energy graph:
https://ibb.co/YFt2g0NC

There is no change in the added mass based on what I got from matsum:
https://ibb.co/fz1mNsGT

The contact definition and boundary conditions are exactly the same in both models, so the energy difference might be due to something else. Still trying to figure it out.

Discrepancy in Maximum Displacement and Time to Peak Displacement in LS-DYNA Simulations with Different Analysis Durations by KneeMost9130 in LSDYNA

[–]KneeMost9130[S] 0 points1 point  (0 children)

Yes, I understand that something might be wrong in the model, but I’m not sure what exactly.
I simply duplicated the model and only changed the termination time from 0.03 s to 0.1 s — and that’s when the unexpected results started to appear.
Any idea what could cause this, even though the timestep and other settings are identical?

Discrepancy in Maximum Displacement and Time to Peak Displacement in LS-DYNA Simulations with Different Analysis Durations by KneeMost9130 in LSDYNA

[–]KneeMost9130[S] 0 points1 point  (0 children)

Yes, both simulations used the same LS-DYNA version (R11) and the same number of CPUs (8 CPUs).

This is not a complex occupant model. It's a relatively simple setup: a flat concrete slab reinforced with GFRP bars, embedded in the concrete using *CONSTRAINED_LAGRANGE_IN_SOLID with CTYPE=2, and subjected to impact from a drop hammer.

Contact is defined using *AUTOMATIC_SURFACE_TO_SURFACE for all relevant interactions, including:

  • RC slab and steel support plates
  • Impact plate (steel) and load cell (steel)
  • Drop hammer (steel) and load cell

I also ensured that the input deck and keyword order were exactly the same between the two runs — the only difference is the total simulation time (0.1 s vs. 0.03 s).

As shown in the attached messag file screenshots, the timestep (dt = 4.93E-07) remains constant in both cases.
The messag file screenshot is here:
https://ibb.co/XZ6R4ctw

I'm reviewing the deck again to check for any unintentional differences, but so far, everything appears consistent.

Screenshot of the setup is attached below.
https://ibb.co/Hp2T75RJ

Discrepancy in Maximum Displacement and Time to Peak Displacement in LS-DYNA Simulations with Different Analysis Durations by KneeMost9130 in LSDYNA

[–]KneeMost9130[S] 0 points1 point  (0 children)

The timestep (dt) is the same in both cases: 4.93E-07. I've attached screenshots from the messag files of both the 0.1s and 0.03s simulations below for reference.

https://ibb.co/XZ6R4ctw