Introduction
MLSS (Mixed Liquor Suspended Solids) are usually expressed in grams per litre (Water online, 2011). Mixed liquor is a mixture of activated sludge, settled sludge, live microbes and even dead cells contained in an aeration basin in the activated sludge treatment. The term MLSS is a general parameter used to design a wastewater treatment plant in the suspended growth process.1 Most of the Sewage Treatment Plant designs were done with the consideration of MLSS as a critical operational parameter.2 Different sewage treatment technologies such as Activated Sludge process (ASP), Sequential batch reactor (SBR), Fluidized air bed bioreactor (FBBR), Moving bed Bioreactor (MBBR), Membrane Bioreactor (MBR) etc., consider MLSS as a vital parameter for their design. MLSS concentrations have a direct impact on viscosity of the waste water.3, 4 Proper concentration of MLSS in the aeration zone can create a healthy environment for microbial survival and improves the settling velocities of the solids.5, 6 This study shows the vitality of MLVSS (Mixed Liquor Volatile Suspended Solids) which tend to be the live microbes facilitating a healthy aeration zone in any sewage treatment plant. Sewage treatment plant has BOD (Biochemical Oxygen Demand) as food and MLVSS as Microbes. Hence, always a healthy F/M (Food to the Microorganism ratio) to be maintained in any sewage treatment plant to have a better oxidation potential.
Food-to-microorganism ratio (F/M Ratio)
The aeration zone of waste water treatment unit is carefully controlled based on the food enters the system and microorganism fed on the food. Here the BOD (Biochemical Oxygen Demand) or COD (Chemical Oxygen Demand) considered to be the food entering the system and Bacteria oxidizes the same in the presence of oxygen. The microbes will most efficiently break down the organic matter in water if they are present in the right proportion.7, 6 Hence, the food to microorganism ratio (F/M ratio) should be maintained properly in the aeration zone. Always a comfortable F/M ratio will be considered for different technologies, since the retention time plays a vital role. Proper retention time helps the microorganisms to serve better in oxidizing the organic matter.1
MLSS and MLVSS
The term mixed liquor suspended solids (MLSS) also can be spelled as Total Suspended Solids (TSS) present in the Aeration zone of a Waste water treatment plant. MLSS test is made to find out the total grams of suspended solids available in the aeration system. This can be determined by using a filter disc which can filter the suspended solids and dried it at 105° C for 1 hour in hot air oven. MLVSS in other hand will help in determining the concentration of volatile suspended solids in the aeration basin. MLVSS is critical in determining the operational behavior and biological inventory of the system. The filter used for MLSS testing is ignited at 550°C for 30 minutes. The weight lost on ignition of the solids represents the volatile solids in the sample. On the whole MLSS is the suspended solids present in the aeration tank which include both organics and inorganic. MLVSS, on the other hand observed to be the volatile portion only, which basically means, this is the portion that are considered to be the microbes.8
Materials and Methods
(Methods: APHA, 2012 Standard Methods for the Examination of Water and Wastewater, 22th Edition. 2540 A, D & E).
Procedure:
Step 1: First weigh the filter paper to get baseline weight. Make sure the filter paper is dry.
Step 2: Filter all 6 samples collected from the aeration tank of different STPs in the weighed filter paper.
Step 3: After filtration allow the filter paper to dry in an oven at 105° C for 1 hour.
Step 4: The dried filter paper is weighed in a weighing balance to obtain the MLSS value in grams.
Step 5: The same filter paper is then dried to ash in 550° C in a Muffle furnace for 30 minutes.
Step 6: Flash off the volatile organic fraction leaving inorganic portion of the MLSS.
Step 7: Now measure the volatile organic portion to obtain the value of MLVSS in grams.
Sewage treatment plant existing
Aeration tank samples from 6 different sewage treatment plants were collected for this study. All the sewage samples were taken from the existing treatment plant in and around the Bangalore location which was erected and commissioned by M/S. Green Enviro Polestar, Pondicherry. The treated water used for toilet flushing and gardening.
Sample 1 Project: Lakshmi Shree group Apartment, Whitefield, Bangalore.
Capacity: 80 KLD (Kilo litres per day) STP.
Technology: Sequential Batch Reactor (SBR)
Sample 2 Project: KERC (Karnataka Electricity Regulatory Commission Building, Bangalore).
Capacity: 100 KLD (Kilo litres per day) STP.
Technology: Sequential Batch Reactor (SBR)
Sample 3 Project: NCBS (National Centre for Biological Science, GKVK Campus Bangalore).
Capacity: 200 KLD (Kilo litres per day) STP.
Technology: Moving Bed Bioreactor (MBBR)
Sample 4 Project: Pyramid Banksia Apartment, Jakkur, Bangalore
Capacity: 280 KLD (Kilo litres per day) STP.
Technology: Moving Bed Bioreactor (MBBR)
Sample 5 Project: NPS (National Public School, Devanahalli, Bangalore)
Capacity: 40 KLD (Kilo litres per day) STP.
Technology: Activated Sludge Process (ASP)
Sample 6 Project: Homely Homes Tranquil Apartment, Hegde Nagar, Bangalore
Capacity: 25 KLD (Kilo litres per day) STP.
Technology: Activated Sludge Process (ASP)
Results and Discussion
The selected waste water samples from 6 different STPs were tested for the content of MLSS and MLVSS as elucidated in (METHODS: APHA, 2012 Standard Methods for the Examination of Water and Wastewater, 22th Edition. 2540 A, D &E). The obtained values of all the samples were tabulated separately. Triplicates of both the MLSS (Mixed Liquor Suspended Solid) and MLVSS (Mixed Liquor Volatile Suspended Solid) values were determined to get a standard result. Mean values were taken and sorted as the standard values for the future research. The results show that 68.85% of MLVSS concentrations available in all the selected STPs total MLSS Value.
Table 1
Sample 1: Lakshmi Shree group apartment, Whitefield, Bangalore
Table 2
Sample 2: Karnataka electricity regulatory commission building, Bangalore)
Table 3
Sample 3: National centre for biological science, GKVK campus, Bangalore)
Table 4
Sample 4: Pyramid banksia apartment, Jakkur, Bangalore
Table 5
Sample 5: National Public School, Devanahalli, Bangalore
Table 6
Sample 6: Homely homes tranquil apartment, Hegde Nagar, Bangalore
Table 7
Average MLVSS % obtained from all samples
|
Description |
Obtained Mean Value of MLVSS % |
Average MLVSS % |
|
Sample 1 |
67.95 |
68.85 % |
|
Sample 2 |
61.29 |
|
|
Sample 3 |
65.94 |
|
|
Sample 4 |
73.15 |
|
|
Sample 5 |
62.48 |
|
|
Sample 6 |
82.32 |
Conclusions
Both MLSS (Mixed Liquor Suspended Solids) and MLVSS (Mixed Liquor Volatile Suspended Solids) plays a critical role in the aeration tank design of any waste water treatment plants. MLVSS preference in aeration tank design can be most effective than the MLSS preference. Although most of the sewage treatment plant designs were made using MLSS value as a base line parameter scale, MLVSS value consideration will give high productivity and consistence. Hence, this study concludes that MLVSS standard value can be considered between 60% to 65% to get a high system efficacy.
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