CHNG5005 WASTEWATER ENGINEERING

INTRODUCTION

A plant producing a wide range of dairy-based products discharges its wastewater to the local sewage system after undergoing on-site primary treatment. As is typical in such plants, the wastewater varies considerably in both flow and quality. The values below show the observed ranges of what is discharged to sewer over a typical week:

Flow: 500 - 700 m³/day

BOD: 350 - 450 mg/L

However, the regional sewage treatment plant is nearing its maximum capacity and the water authority is insisting that the plant's wastewater be further treated on-site before it is discharged to sewer. Thus, the company has recently been advised that any future discharge should have a maximum BOD level of 5 mg/L. Your consulting company has been contracted to carry out preliminary sizing calculations for an on-site secondary treatment plant.

As is often the case in such consulting work, this (two part) project will be carried out in small groups. In any design process, there is rarely a single correct answer. An important aspect of engineering design is the ability to make realistic assumptions, as the assumptions you make will invariably influence the answers you get. Thus, it is essential that for both parts of this project you make it clear what assumptions you have made and why you think that these are reasonable.

PHASE 1: SUBSTRATE REMOVAL KINETICS

An Internet search provides laboratory data for wastewater from a skimmed milk plant that is being successfully treated in an aerobic digester where the volatile suspended solids (VSS) has been measured ataround3000 mg/L.

The biomass growth rate (r_x in mg VSS/L.hr) was found to be adequately described by the following equation: where S is the substrate level (in mg BOD per L). The yield (Y) was reported to be 0.6 mg VSS/mg BOD.

Question 1

Sketch the behaviour of the specific growth rate (rx in hr^-1) as a function of S over the likely range of interest (i.e. S up to 500 mg/L).

Question 2

Calculate the biomass growth rate (r_x in mg VSS/L.hr) when S = 5, 50 and 250 mg/Lfor the measured VSS loading. Note that the biomass loading (X in mg/L) can be taken as equivalent to the VSS loading in an aerobic reactor.

Question 3

Comment on the importance of the endogenous respiration term in the published expression for .

PHASE 2: INITIAL MASS BALANCE CALCULATIONS

A basic aerobic digestion system consists of the following units:

• A mixer (to combine the fresh feed with recycled activated sludge).
• An aerated digester.
• A clarifier(producing a treated wastewater overflow and a sludge underflow).
• A splitter (producing a recycled sludge stream and a wasted sludge stream).

For preliminary calculations, the following assumptions may be made:

• The digester is a single, well-mixed reactor (i.e.essentially a CSTR).
• Substrate consumption (and thus biomass generation) only occurs in the digester.
• Substrate consumption can be adequately described by Monod kinetics with no rate restrictions imposed by oxygen transfer limitations.
• The clarifier operates perfectly (i.e. all biomass in the feed to the clarifier reports to the underflow stream).

Question 4

Discuss how reasonable each of the above assumptions is likely to be for a real plant.

Question 5

Such systems are typically operated so that F:M (i.e. the food to microorganism ratio or the sludge loading rate) falls in the range 0.2 - 0.5. Explain the consequences of operating outside this accepted range.

Question 6

Using the above assumptions, develop a set of mass balance equations that describe this basic aerobic digestion system. Clearly note any additional assumptions that you make. Also indicate which variables in your model are known and which variables must be calculated by solving your set of model equations.

It is strongly advised that at this point you solve your mass balance model by hand. This will allow you to see whether your model gives reasonable answers (noting that a common mistake in any model development exercise is not checking that the units for all variables are consistent), as well as providing values that can be used to check your computer (i.e. Excel) solution.

Question 7

Develop an Excel spreadsheet that solves your mass balance model using Solver. Your modelshould give the substrate level (in mg BOD/L) and the biomass level (in mg VSS/L) at all key points around the circuit, as well as the required digester volume (in m³), and the hydraulic and solids retention times (in days).

Your spreadsheet should have the following characteristics:

• It should allow the user to readily change any operational variable or circuit parameter.
• It should make clear how the mass balance equations are solved, noting that in various scenarios it will be necessary to solve the model for different sets of variables.
• In terms of the above considerations, the spreadsheet should be 'nicely' formatted so as to assist external assessment.

Solving your model will require you to determine (or set) values for the circuit parameters ?, ? and ?. The first two determine the recycle activated sludge flowrate or RAS (i.e. ?Q where Q is the feed flowrate to the circuit) and the wasted activated sludge flowrate or WAS (i.e.?Q), while ? is the fraction of biomass entering the secondary clarifier that reports to the underflow stream). At this stage, you should set ? = 1 (i.e. assume that you have a perfect clarifier).

Note that an alternative to setting is to provide a value for the biomass concentration in the overflow from the clarifier.

Explain clearly how values were determined for ? and ?.

Question 8

Use your Excel spreadsheet model to examine the sensitivity of this aerobic digester circuit to the parameters ?, ? and & ?. In looking at such sensitivity issues, it may be useful to define a 'base case' set of conditions around which changes are then made.

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