DESIGN OBJECTIVES
Your goal is to find the best solution to meet the Client’s wastewater treatment needs over the next 20 years. How do you design a solution that optimally achieves the goals & objectives of the project?
1. First, you must first define the goals & objectives.
GOALS should reflect the ultimate purposes of those who have both direct and indirect interests in the project.
example: to produce safe effluent water that protects human health and the environment
OBJECTIVES specify which characteristics of the system are to be optimized in order to achieve the goals. Things such as cost, aesthetics, etc.
ecample: maximize the safety of the plant
2. Then specify the criteria against which you will measure how well the goals/objectives are met
CONSTRAINTS: drive the alternatives selection and design by mandating limits that cannot be exceeded
these MAY be legal limits
examples: must fit the WWRF on the available land
must comply with NPDES permit limits from the state
must comply with water rights
maximum amount of money (potentially)
CRITERIA: are desirable or undesirable elements against which the design will be judged;
a set of parameters used to measure how optimum a solution is with respect to the objectives
Criteria are subject to interpretation !
As the design team you need to balance your professional judgement with both the Client concerns and public concerns.
Non-technical issues have a large impact on the final design
social, economic, & political issues should be considered
Frequently, the goals, objectives, and criteria may conflict; therefore, it is important to rank the relative importance of each. Example: optimal water quality vs low cost
How well each alternative satisfies a criteria is frequently a subjective judgement, since it doesn’t involve quantifiable elements
Due to the complexity of our problem, and the HUGE range of possible combinations of processes to achieve our goals, your team will likely need to “discretize” the problems.
Example A
1. primary treatment of all WW flow
2. secondary treatment of all WW flow
3. advanced treatment of all WW flow
for next 5 years, remove ammonia
in 10 to 15 yrs, need to remove N
in 15 to 20 yrs, need to remove P
Example B
1. meet Q and effluent limits for next 5 years
2. meet Q and effluent limits for 5-15 yrs
3. meet Q and effluent limits for 15-20 yrs
Example C
1. remove solids
2. remove BOD
3. remove bacteria
4. remove ammonia
5. remove N & P (for reuse, or in 15-20 yrs)
6. treat biosolids
Example D: unit processes
1. primary settlers
2. activated sludge
3. biotrickling filters
etc.....
** first cut: eliminate things that don’t meet CONSTRAINTS
** next: primary criteria? main criteria with sub-criteria?
-> try to narrow to 3 to 5 options for detailed evaluation (1/person)
** listing the criteria is not enough -- need to include a sentence or 2 definition, so that everyone will interpret each criteria the same
COMMUNICATION IS VITAL TO DEFINE AND WEIGHT DESIGN OBJECTIVES
1. NORMAL
1. EXTERNAL - design engineers with clients, stakeholders
* meetings & presentations
* town meetings with presentations & Q/A
* workshops (1-2 days)
- written via surveys, letters, e-mail
2. INTERNAL - within design firm or team
* brainstorming
* informal presentations
* exchange written proposal, sketches, calcs
* circulate among whole team
2. CONFLICT RESOLUTION
1. EXTERNAL
- due to miscommunication
(most conflict can be avoided by good, open communication)
- client or engineer making criteria into constraints
(not enough flexibility)
- ethical
- may require a 3rd party mediator to resolve
DECISION MATRIX AN EFFECTIVE TOOL TO COMPARE & PRESENT RESULTS
CAN RUN A COST:BENEFIT ANALYSIS FOR EACH ALTERNATIVE; PICK ALTERNATIVE WITH HIGHEST B/C RATIO
Examples of constraints and criteria applicable for wastewater treatment plants
climate (constraint?) / temp affects rxn rates, freezing conditions can affect phys operation
meet permit (constraint)
exceed – criteria / BOD, TSS, DO, coliform, pH,
ammonia, NO3, NO2, P,...
public acceptance / aesthetic appearance, odor, traffic, etc.
odor / potential to generate odor; uncontrolled releases
past experience / applied at other WWTPs
requires pilot testing / not enough information available to design and estimate performance without pilot tests
environmental impact / surface water, groundwater, biosolids, ecosystems
plant personnel capable of operating / already trained; more training needed, more personnel needed
energy conservation / due to energy costs; to conserve
capital cost / initial costs to construct the process on site
O&M cost / yearly operation and maintenance costs
complexity / how difficult to operate under routine conditions and under shock loads; difficulty fixing when broken;
ease of construction
flexibility / process can accommodate longterm change in influent quantity, quality, or regulatory requirement by adjusting operating parameters
reliability / tendency for minimal mechanical equipment failure resulting in down time;
stability / stability of process under short-term stressed conditions; ease of correcting upsets
chemical requirements / either desire or do not desire to use chemicals in the treatment processes
safety / to operators, public
residuals / handling, cost, beneficial use
modularity / easy to upgrade with similar processes due to modular design; take one portion out of service for repair while maintaining operation of others
use of existing facilities / does not abandon existing facilities but utilizes them to the optimal extent
negative impacts to existing processes / will not require changes in existing upstream or downstream processes
serving water treatment plant & industries / handling chemical sludges from drinking water treatment; industrial wastewater flows/quality