Environmentally Friendly Waste System

We called the vacuum truck to come and load the digester with waste in the morning. We loaded it all the way full to the third sampling valve. We took a 200ml sample of the sludge from the bottom valve for it to be more representative. From there, we proceed to the farm for testing.


We spent quite a large amount of time in the lab trying to find the reagents as it was quite messy. We finally managed to sort out the mess and did the testing. It was our first time performing the experiment and these are our results

Sludge weight: 52.1432g

pH: 10.62

Volatile Acids: 19mg/L

Dissolved Oxygen: 2.58ppm

Chemical Oxygen Demand: 156mg/L

Nitrogen: 20mg/L

Phosphorous: 2.1mg/L

Potassium: (Have not completed, insufficient reagents. Reagents to be obtained second week of October)

We met up with Dr Han and Ireaneus at Ensure Engineering. We went through the prototype design with him and started to piece together the little details regarding our designs. We talked about the type of equipments needed, their cost and the materials of construction.


We came up with a 1.1m tall design of the digester. Made out of a transparent plastic (ask what plastic) that can withstand the heat from the heater in the aerobic stage of the digestion.

One problem that arises today was the difficulty in getting the materials needed. We calculated that the prototype required a circular base of diameter, 20cm for the height to be 1m. However, this is not do-able in practice because in the market, the largest circular base on offer is 16cm. Which means our prototype would have to be nearly 2m to achieve the same efficiency as opposed to it being 1m with a 20cm diameter base. Hence, we made the decision to change it from a circular design to a square design. The square based used is 20cm by 20cm. Allowing us to continue building the prototype at 1.1m.

Finally we settled that the height of each containment should be 37.5cm and the space below to collect sample and the space in between the containment should each be measuring 20cm in height. Thus, giving us a total height of 1.15m

We met Dr Han to finalise the details of the digester. We have the drawings of the final design at the bottom:

(photos to be added all at once)

There are a few differences in design of the prototype in comparison to the actual digester. Firstly, there is the addition of stirrers at both stages to homogenize the sludge. Also, the prototype design consists of a thermostat to control the temperature in the aerobic stage which makes it more convenient. Then we also incorporated the methane capture section of the design. A vacuum pump is needed to ensure there is a pressure difference and hence allowing the biogas to be transferred to the methane capture device for separation. Another minor difference is the incorporation of gas test sockets and pressure gauges to allow easier reading.

In the methane capture device, the biogas is passed through an activated carbon filter before being bubbled into water or an organic solvent (NaOH) this would ensure that the carbon dioxide and the hydrogen sulphide is removed from the biogas and pure methane can be trapped. In the design, there is a pipe leading out from the solvent containment to allow for drainage.

He also introduced us to the project supervisor, Ireaneus. He is to guide us through the whole building of the prototype and handle all the technical issues.

At the moment we have found the drager tube for the testing of CO2 and H2S. However, we have not been able to find a suitable drager tube for CH4. The drager tube for CO2 and H2S cost about SGD $156 and SGD$103 per pack of ten respectively. When we called the supplier to enquire about the drager tubes for CH4, he said that he does not sell drager tubes, but he does supply another brand called Gastec. It costs slightly more than SGD $80 per pack of ten. In addition, this particular methane detection tube would require the use of a pump which is over a hundred dollars.


The measuring range for CO2 drager tube is: 5-60 vol%

The measuring range for H2S drager tube is 1-60ppm


The working principle: a measured volume of gas (or air) is drawn through a tube which contains chemicals which change in color in response to the presence of a specific target gas (or range of gases) present in the sample.



To select the appropriate gas detection tube you need to know what gas or gases is/are to be detected, and at what probable concentrations the gas may be present, or at what level of exposure the test is to be conducted.


Each dragger tube also has a specific number of strokes required. Hence, it is important to take note of this before doing any measurements. After conducting measurements, the drager tubes should be flushed.



The "outlet" end of the detector tube is inserted into the gas collecting pump. The "inlet" end of the tube is exposed to the air to be tested, and the pump is operated for the required number of strokes before looking for a color change on the tube's gas concentration scale.