VVAN25 Wastewater treatment part

F1-8 +14

PF

peaking factor:Ratio of a maximum flow to the average flow. This can be based on population or flow.

SVI

Sludge volume index - volume occupied by one gram of particles after setting

Sludge load

Load of biomass TO the sludge, also F/M - BOD/kg VSS per day

The first steps of the preliminary and primary treatment in WWTP

First water is pumped and flow measured. Then screens remove larger objects. Then the grit chamber removes sand, pebbles etc. Also called sand trap. Then there is the primary settling removing up to 30% BOD

MLSS/MLVSS

Mixed liquor (volatile) suspended solids. Is the concentration of suspended solids (SS) in activated sludge basin, 1.5-5 g/l (or VSS)

WAS

stands for waste activated sludge. Sludge that is discarded and not returned in RAS

MBR

Membrane bioreactor. Is a combination of activated sludge process and membrane filtration.

What is a trickling filter process and how does it work?

Not a filtering or sieving process. Works by pushing up water, up into rotating arms that distribute the sewage water onto a media where bacteria grow on the surface - biofilms. The bacteria process the waste water in the same manner as in suspended growth processes but they are attached instead. It is important that we always have a upward flow of the water, it is also this flow that drives the distributors (rotating arms). When the biofilm grow thick there can be some shearing off, which creates a little bit of sludge. Therefore a subsequent treating process is required.

Why do we have recirculation of water in the trickling filter process? In which ways can we recirculate it?

• Evens out variations in flow

• Increase flow through filter bed

• BOD removal enhanced

• Prevent stopping of rotating arm

• Keeps the biofilms continuously wet (avoid drying out during nights)

We can have direct recycle - water is taken directly from the outlet from the trickling filter back to the inlet of the filter.

Another alternative: return flow from final clarifier to primary clarifier. THis requires that the primary clarifier can take care of the extra sludge that is added.

Advantages/disadvantages TF

Advantages: easy to operate, can stand peak flows, low sludge production, low foot print. Disadvantages: hard to control/optimize. Limited efficiency. There can be problems with odour and flies from the filters..

RBC

Rotating biological contactors. Filters rotating in water

MBBR

Moving bed biofilm reactor. A mix between the activated sludge and the attached growth process. Uses small plastic elements in an activated sludge reactor. Does not use return sludge flow.

IFAS

Integrated Fixed-film Activated Sludge. Operated as an activated sludge system but we have a surface with filters. Optimized because some microorganisms grow on surfaces, other does not.

How can we approximate the BOD5 in suspended solids (particulate BOD)

Particulate BOD5 can be estimated with 60-70% of SS. One could also say that 30% or BOD is the soluable part.

relationship between sludge age & temperature.

Higher temperature, lower sludge age is required compared to a colder environment where longer sludge age is required. This is important when designing the secondary treatment. 

In an aerated tank with activated sludge, where in the tank will the nitrification rate increase and why?

Nitrification rate increases when the BOD decreases due to nitrifying bacteria being able to compete then

What is required for the denitryfying process? How are these requirements fulfilled in a pre-denitrification vs post-nitrification step?

anoxic conditions, enough BOD or another carbon source, nitrate NO3. Pre: low NO3, high BOD. Nitrate must be recycled from aerated basin. Post: high NO3, low BOD. Might need to add carbon source.

EBPR

enhanced biological phosphorus removal

describe how mixing should occur in chemical precipitation

rapid mixing at first, to create contact between particles and coaugulant. Then slow mixing so that flocs do not break but can continue to grow

Pre-precipitation - advantages, disadvantages

Addition of chemicals before secondary treatment, after the sand trap.

• Requires lots of chemicals

• Removes up to 90% of P

  • Good but there must be some P left for the secondary treatment step

• Also removes lots of BOD

  • Good if small bioprocess,

  • good if you want high sludge age (due to less sludge production)

direct precipitation

Is the same as pre-precipitation but there are no secondary treatment step in process

Simultaneous precipitation

chemical agent is added to the aeration basin. No extra basins required. Results in Increased sludge production leads to low sludge age and chemicals in our sludge that we usually don’t want. Good for focus on p-removal

post precipitation

• Chemical precipitation step after the biological treatment step - after the secondary clarifier

• Extra space/basins required

• High treatment efficiency

• Tertiary sludge called chemical sludge.

What are the requirements of bio-p process

• Altering anaerobic - aerobic (or anoxic) conditions. First anaerobic and then aerobic/anoxic conditions

• VFAs (volatile fatty acids) in anaerobic zone

• Removal of sludge

• Not too long storage of sludge in sedimentation basin

  • If we along the way after the aerobic zone will have anaerobic conditions, then we’ll have phosphorous release which we dont want

Bio -P: In water phase what happens to orto-P and VFAs concentration in anaerobic phase and then in the next anoxic/aerobic phase? Likewise, what happens to PHB and poly-p in the biomass in the two phases.

Water 1st phase: orto-p increases, VFAS decreases.

Biomass 1st phase: PHB increases, poly-p decreases

2nd phase

Water: orto-p decreases

Biomass: Poly-p increases, PHB decreases

How do you combine bio-p and activated sludge process?

One can have an anaerobic tank before the anoxic tank. The P release will occur in that tank. Then in the next tank dinitrification occurs (no Bio-p). Then the uptake of p occurs on aerobic tank. It is important that no nitrate is recycled to the anaerobic tank, so the RAS goes to the anoxic tank. However one can have a second internal cycle, where water from the Anoxic denitrification tank is recycled into the AN tank, which will return residual carbon and hopefully contain low NO3 concentration

Give examples of separation techniques aside from traditional settling basins

lamella sedimentation (mostly DWT), ballasted flocculation (actiflow), DAF (dissolved air flotation)

Which are the steps in sludge treatment?

total hardness

Mg + Ca

Which are the most common treatment steps in DWP for surface water?

sievning, chemical precipitation + separation, rapid filtration, slow sand filtration, disinfection with UV or chlorination, adjustment of alkalinity and pH.

Which are the most common treatment steps in DWP for groundwater?

aeration/oxidation to remove Fe and Mn and other gases, rapid filtration, softening, adjustning pH/alkalinity, disinfection, sludge treatment

How do we remove hardness in water?

First we add lime to remove the carbonate hardness. Then one can add soda ash to remove the non-carbonate hardness as well if that is required.

Why is disinfectant by-products a risk? Which disinfectant by-products are there?

disinfectant by-products are a risk becauase they can be carcinogenic. Chlorination can lead to formation of THMs. Ozonation can lead to the formation of bromate.

which three main steps are there in treatment of WWTP sludge. How DWT sludge treatment differ from this?

thickening - removing free water, stabilisation - stop biologcal activity (break down organic matter), dewatering (remove as much water as possible). DWT Sludge does not need stabilization due to its low organic content

Mention some thickening processes

gravity thickening (circular basin with rotating arms), flotation thickening (air injected from below, sludge floats up, is collected), gravity belt thickening (sludge transported on a belt, water drains through it)

Describe a stabilization of sludge process

a common method is anaerobic digestion. oxygen free environment, long retention time, heating (37/55degrees) leads to biogas

mention some dewatering processes

centrifuges, filter press, drying beds

what can lime be used for in a WWTP?

can be used in stabilisation for sludge

what is conditioning in wwtp?

a step that is often needed before dewatering of the sludge. usually adding chemicals to improve coagulation

how can DWT-sludge be dewatered in a non mechanical way?

sand drying beds or lagoons

nitrification/denitrification when it comes to alkalinity

nitrification consumes alkalinity while denitrification produces (some)

which % of BOD removal can one assume in primary settling?

30%

which particle characteristics are important for function of separation processes?

size, density, shape, charge, floc strength

for dimentsion of sedimentation basins hydraulic surface loading rate is used. for secondary clarifiers this might not be enough, what else could be important and what is the difference to primary clarifiers?

Solids loading rate. The difference is that the SS-concentration is much higher and

that interactions between flocs starts to play an important role.

For a rapid filter, what happens with turbidity and headloss during a filter run?

First, one will have a increase in turbidity, the clean filter will let through smaller particles. Then, as particles are stuck in the filter, turbidty decreases as smaller particles are filtered out by the other particles in the filter. The process of decreased turbidity is called filter ripening. During this whole time the headloss increases and when it gets too high, backwashing is conducted and a new filtration cycle begins

sludge age

The ratio between the existing amount of sludge in the biological system and the amount of sludge removed from the system each day

what do we mean with yield constant in AS (kgSS/kg reduced BOD)

The yield constant is a measure of how much sludge the reduced amounts of BOD give rise

to. So kg total sludge production/kg removed BOD

suggest three different process alternatives for polishing the effluent from the secondary clarifier

• Sand filtration with or without chemical pre-treatment

• Dissolved air flotation (DAF) with or without chemical pre-treatment

• Settling (with or without lamellas) with chemical pre-treatment

Extending an existing TF process with an AS reactor, how should you design it and why

the aerobic zone can come before the anoxic zone since the water will be low in BOD and a carbon source is required to be added anyways.