University of Pretoria Process Engineering Associate Excel Worksheet
Process Engineering Associate – Pre interview questionsWaste Conversion LTD is a company that treats organic waste using worms to form organic
fertilizer and an insect based animal feed. In this simplified process, different types of organic
waste (cabbages, bananas, tomatoes, manure…etc) are delivered to the factory using trucks.
As a pre-processing step, an industrial waste mixer is used to mix the waste at different
predetermined compositions to form several recipe batches. These waste recipe batches are A,
B, C and E.
Depending on the production plan, a predetermined number of beds (with unique identifiers) are
loaded with the batches of waste at different recipes. Typically, one bed is loaded with one
mixed waste batch that weighs 1500 kilograms. Thereafter, young/small worms at either of two
loading densities (17.8 kg or 32 kg) are added to the beds and left to feed on the waste for a
specified amount of days. After the days have passed, the grown/adult worms are harvested
from the beds and weighed – from which growth factors are calculated. The remaining
waste/compost is routed to the next processing step.
Please use the data sheet provided (excel workbook) labelled “Data sheet” to answer the
1. Please review the database (excel workbook) provided and do the following:
a. Using measures of central tendencies, develop a dashboard on MS Excel or
Google Sheets showing both weekly and monthly production performance at
loading densities of 17.8 kgs and 32 kgs with regard to: (1) Growth factors and
(2) Feed yields. In this analysis, clearly distinguish between production beds with
a cycle time below 17 days and those with cycle times above 18 days.
b. Improve on the above by developing a control chart displaying daily production
performance with regard to growth factors for beds with cycle times below 11
days at loading densities of 17.8 kgs. Include upper and lower control limits to the
2. Typically, waste mixed with the recipe A composition is used owing to higher historical
yields. However, during the production period (Jan to October 2018), it was noted that
the worm growth factor varied significantly when recipes B, C and E were used at
different loading densities (17.8 kg and 32 kg). As the quality engineer, please use
analysis of variance or other known statistical methods to answer the following pending
questions (using the data in the spreadsheet):
a. Is it best to use recipe A for all beds at any loading density?
b. In the event that recipe A is not available, what recipes (between B, C and E) do
you recommend using and at what loading density? Please give these in
descending order based on growth factors.
Please note that the data is not clean i.e. there are outliers as well as missing data points. All
definitions have been provided in the data sheet (excel workbook)
Please provide your answers/responses in the MS Excel with referenced calculations/analyses
where necessary. If successful, note that you will be required to interpret your responses in