Solids

Theory

The solids content is probably the most important factor in wastewater. There are different solids classifications in wastewater depending on their size and also on the treatment these solids go through in the laboratory. /7/

The types of solids used during this laboratory work were the TS (Total Solids), MLSS (Mixed Liquor Suspended Solids), and MLVSS (Mixed Liquor Volatile Suspended Solids). The TS is the residue that remains after a sample of wastewater is dried under ~105 whereas the MLSS is the residue that remains on a filter with a specific pore size and dried under ~105 . The MLVSS shows the amount of solids that volatilize after the MLSS are burned inside a muffler at ~550 . /

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Measurements

The sludge taken (also referred to as “wasted” in everyday laboratory language) from the clarifier and oxygen tanks to keep the SRT (Solid Retention Time) under a certain limit has been used to run this test. The amount being taken depends on the desired SRT and can be easily calculated with a formula, which is illustrated

later on.

The filters to be used, in this case GF/A with 1,6μmpore size manufactured by Whatmann, need to be muffled. To do that they will have to be kept for an hour inside an oven at ~550 and stored the whole time inside a desiccator. The weight of the clean filters needs to be known in order to calculate the MLSS.

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2 ml samples from the oxic and from the settler were filtered using these filters, which are then kept for roughly an hour inside an oven at ~105 temperature.

Aluminum dishes can and should be used to support the filters. After that the filters are kept for a while inside a desiccator until they reach room temperature before weighting them again. With the weight obtained a formula has been used to

calculate the MLSS:

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( )

a is the mass of the filter after the filtration, in milligrams b is the mass of the filter before the filtration, in milligrams V is the volume of the sample, in milliliters.

Example:

After this stage the filters were then kept again for roughly an hour inside a muffler at ~550 . Again they were allowed to reach room temperature afterwards so that their weight could be measured. With the value thus obtained the following formula has been used to calculate the MLVSS:

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d is the mass of the filter and residue, in milligrams

b is the mass of the filter before the filtration, in milligrams V is the volume of the sample, in milliliters.

Example:

The MLVSS of the oxic sample is used to calculate the SRT of the pilot using the

following method:

First the amount of MLVSS (mg) taken out from the two tanks (oxic and settler) needed to be calculated separately and then added together to get the total amount of MLVSS taken out from the pilot. Next another formula is used to calculate the amount of MLVSS present inside the oxic and settler tanks, which is essential, together with the MLVSS of the oxic sample, to finally calculate the SRT. The following steps have been used:

Step 1: Amount of MLVSS (mg) taken out from the oxic tank

( )

^{mg MLVSS}

( )

^{mg amounttakenfrom oxic}

(

^l

MLVSS_TakenOxic = _OxicSample ⋅

)

Example:

( )

mg mg l mg

MLVSS_TakenOxic =3500 ⋅0,300 =1050

Step 2: Amount of MLVSS (mg) taken out from the settler tank

( )

^{mg MLVSS}

( )

^{mg amounttakenfromsettler}

(

^l

MLVSS_TakenSettler = _{SettlerSample} ⋅

)

Example:

( )

mg mg l mg

MLVSS_TakenSettler =5100 ⋅0,500 =2550

These 2 values are then added together to find the total amount of MLVSS taken out from the pilot. Finally we need the total amount of MLVSS present inside the pilot and then use it, together with the last value obtained, to find the SRT. These are the steps:

Step 1: Amount of MLVSS (g) present in the pilot

( )

mg MLVSS

( )

mg Totalvolumeof OxicandSettler

( )

l MLVSS_Pilot = _OxicSample ⋅

Example:

The MLVSS taken out of the reactor can be controlled by getting a known amount of waste water and the SRT in turn can be adjusted to the desired value and kept that way as long as required during the experiment period.

Discussion

Any support or container that can sustain high temperatures could be used to support the filters during handling, like for example crucibles made of ceramic.

However it is a better idea to use aluminum dishes (the ones used for baking muffins should do) since being a metal it cools down faster inside the desiccator. In fact just after roughly 30 minutes inside the desiccator the dishes can already be taken out and the filters weighed.

The aluminum containers and the clean filter should be muffled beforehand and stored inside a desiccator to ensure that the MLVSS is correct. This is because the clean filters and the aluminum dishes out of the box have some impurities on them that will affect greatly the values once weighed. It is also extremely important to never touch the dishes or the filters with bare hands otherwise they will have to be

muffled again.

The sample from the settler should be taken from the recirculation tube that goes back to the anoxic tank. In case the speed of the pump could be set higher for this purpose. This is to ensure that the sample is homogeneous. It is also a good idea to homogenize the samples using a domestic blender

In document A guide for building, operating, controlling and maintaining a complete mix activated sludge pilot reactor (sivua 19-23)