1. increasing population and relatively higher waste

1. Introduction

 

Municipal solid waste is a major
challenge in urban areas. Decreasing availability of land coupled with ever
increasing population and relatively higher waste generation per capita leaves
few options for the treatment of solid waste. In contemporary times, direct
waste land filling is nearly impossible considering space constraints and it is
also prohibited to do so in some countries. Thermal treatment of solid waste is
a possibility. However, the process in cost intensive and may not be affordable
by many municipalities. Mechanical biological waste treatment such as
composting and fermentation is an alternative that can be utilized to deal with
solid waste management. Designing a facility wherein mixed municipal waste can
be treated, such as bio waste and garden waste, to produce valuable products at
the end of the process train is not only an efficient way of dealing with
waste, but also drives optimum return on investment to fund such a waste
treatment plant. (Christensen, 2011)

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This report is going to present
the design of a mixed municipal solid waste plant wherein two different types
of solid waste, namely bio waste and garden waste, are converted into useful
products such as compost and pure biogas. Arriving at the waste quantity is the
starting point of this design which shall facilitate the dimensioning of
storage unit. It will be followed by illustration of different processes and
unit operations through process flow diagram. Mass balance will also be
presented to understand the material flow across the entire plant. These
calculations shall be used to determine various equipment that are going to be
used as well as a suitable plant layout. Basic overview on cost analysis is
presented in the end to obtain an idea on feasibility of this project. Ground
plan and calculations are included in the annex of this report.

 1. Waste Storage UnitThe first stage of waste treatment comprises of Bio-waste and garden waste storage. Bio-waste and garden waste are received and managed separately at this stage. It is necessary to have a dedicated storage space for waste to ensure a continuous supply of feed, which is the municipal waste, to the plant and avoid any disruption or halt in operations that may be caused otherwise. Incoming waste is brought into the reception area where each loaded truck is weighed and registered before entering the facility.  Three different entry points with reception and registration are considered to accommodate multiple vehicular entries into the plant simultaneously. This is also necessary to evade any congestion at the entrance of the facility.A separate storage area is allocated for the two different types of waste wherein weighed and registered delivery trucks unload bio-waste and garden waste respectively.  There is a possibility of foul smell being emitted should the waste be piled up on open space and in order to minimize this issue as well as maintain hygienic conditions within the plant, construction of closed area is to be done for storing both bio waste and garden waste indoors. From the data given in contract, the total quantity of bio-waste and garden waste is calculated. Seasonal variations of waste are considered rather than an average values of bio-waste and garden waste throughout the year. The data of the city of Kaunas, Lithuania 2009 are used and tons of waste generated for primarily four seasons – spring, summer, autumn, and winter – are calculated. The results are as shown in Table 1. (Denafas et al., 2014)Table 1: Total yearly waste and seasonal variationsBio-waste (Tons)Garden waste (Tons)Spring 16922.51155.16Summer22445.94372.42Autumn 24407.16263.79Winter16274.11108.63Total waste per annum80049.71900To size the storage facility, it is important to estimate the daily peak flow of both types of waste. This can be calculated by averaging the incoming waste throughout each season and arrive at the volume of waste for each day. Following which, the peak volume is now considered to select the dimensions of waste storage area. The results are depicted in Table 2. Table 2: Tons and volume of waste generated per dayBio-waste Garden waste Daily peak weight (tons)268.214.05Daily peak volume (m3)670.5326.99Density (tons/m3)0.40.15The storage unit is designed to store bio-waste and garden waste for a maximum period of two and five days respectively. The peak volume calculated is used to size the storage facility and dimensions are tabulated in Table 3.Table 3: Dimensions of storage facilityBio-waste Storage Facility Garden waste Storage FacilityLength (m)404Breadth (m)9 9Height (m)4 4Area (m2)360 36Volume (m3)14401442. Pre-treatment UnitThe aim of including the pre-treatment unit operations is to eliminate the unwanted substances present in the incoming waste. As mentioned in the contract, bio-waste is expected to have impurities between 2.6% to 5.4% and the content of it in garden waste is estimated around 4.5%. The separation of these impurities is vital to maintaining end products of uniform quality which may otherwise cause disruptions in downstream processes, disturb the fermentation process or affect the overall quality of compost. The technologies available for pre-treatment in the market are very specific and the luxury of a large variety of options is not available, as in the case of other industries. This is because waste management industry in itself is in its nascent stage and the concepts of waste management have picked up momentum only in the recent past. In our pre-treatment unit, we’re considering processes primarily for the purpose of separation and size reduction and to achieve this, we are selecting machinery which is well proven and belong to the category of best available technologies for their respective operations. There are two possibilities of treating two different types of incoming waste, i.e. bio-waste and garden waste. One way to do this has two separate treatment lines for each category of waste. However, it is more appropriate to choose this technique when the plant is treating a higher quantity of garden waste. In this case, daily peak flow of garden waste is about 1.48% of overall peak daily waste (4.04 tons out of 272.26 tons) and it makes much more sense to first treat bio-waste followed by garden waste using the same pre-treatment machines. So, for this proposed plant design, one single line treatment for both types of incoming waste is chosen. This has quite a few advantages over two line treatment system. For example, space, power consumption and extra investment on equipment can be saved. 2.1 Pre-treatment Process Flow To remove ferrous metals     To open larger plastic bags     Magnetic SeparationBag OpenerIncoming WasteDensity SeparatorTrommel ScreenFine ShredderTo Fermenter /CompostTo separate remains of plastics, paper, film from upstream processes       Larger particles separation from finer particles       Figure 1: Pre-treatment Process Flow DiagramThe complete single-line pre-treatment processes are depicted in Figure 1. Bio-waste and garden waste are treated in batches one after the other using same conveyor belt systems and equipment. Waste from the storage unit is loaded using wheel loaders into feeders which feed the waste onto conveyors transporting waste to the processing unit. The description of equipment involved in carrying out above mentioned operations is discussed in section 2.2 in detail.The processing unit comprises of the following processes – Bag opening Magnetic separationScreening Air separationFine shredding2.2 Pre-treatment EquipmentThis section comprises of descriptions and specifications of all technical equipment involved in the pre-treatment process.2.2.1 Bag OpenerThe possibility of waste coming in plastic bags facilitates the inclusion of a bag opener at the start of pre-treatment process. Albeit manual checking is common in most of the waste treatment plants, particularly in developing countries where cheap manpower is available, it is planned to eliminate this considering state-of-the-art nature and location of the proposed waste treatment plant. The bag opener decided upon for this plant is BOS 4000 which shall be supplied by a sub-vendor namely Environmental Marketing Solutions Ltd. and it has the following specifications.Table 4: Bag opener specificationsEquipment DetailsLength 3.972 mBreadth 1.999 mHeight2.781 mThroughput per hour35 tonsFigure 2: Bag opener(EMS Turnkey Waste Recycling Solutions, 2018)The above equipment yields an opening efficiency of over 95% according to claims of the supplier and is effective in opening both wholly as well as partially bagged waste. It consists of special ripper knives to work upon even the smaller bags. The bags present in the waste stream are opened and the product is conveyed to the downstream processes using a chain conveyor in the discharge hopper. (EMS Turnkey Waste Recycling Solutions, 2018)2.2.2 Magnetic SeparatorSome of the impurities in waste stream are expected to be metals. Thus, a magnetic separator is incorporated to remove ferrous metals from the waste stream. It is also important to remove the metals in order to protect the down-stream sizing equipment, i.e. shredder, from being harmed. Out of several options, an over-head magnetic separator is chosen for this process because of its flexibility, simple construction and overall good performance. The working principle is quite simple and is depicted in figure 3. A permanent magnet is installed at a fixed distance, 900 mm in this case, from the conveyor belt with waste stream having a working width of 2 meters.  All ferrous metals being conveyed along with the waste stream are attracted to the magnetic field generated by the over-head magnet and segregate from rest of the waste. In this plant, the permanent magnet is proposed to be positioned over pulley instead of over belt in the conveyor system as the metal tramps are removed easily when magnets are placed over pulley because this position takes the aid of the natural break-up of the metal flow as it leaves the head pulley. Thus, a significant performance improvement is reported whenever a magnet is positioned this way. (Steinert GmbH, 2015)Over-head MagnetFigure 3: Magnetic SeparatorMagnetic FieldFerrous materialsConveyor beltNon-ferrous materials(Steinert GmbH, 2015)2.2.3 Trommel ScreenA trommel is included in the processing unit to perform screening of the waste based on their particle size. It is used to sort the waste as per size requirements and is nothing but a rotating drum. It has been used successfully in numerous waste recycling plants around the world and hence, is an established equipment to be used for pre-treatment of waste. Its downside of occupying more footprint, the intermittent formation of plaits and dust emissions are balanced by the advantages of decompressions, thorough mixing, minimal risk of clogging and it is a rugged, heavy-duty system in general. A basic working principle of a trommel screen can be observed in figure 4.Figure 4: A typical trommel screen                  (Christensen, 2011)A drum screen such as trommel can handle municipal waste effectively and desired size separation can be obtained by selecting appropriate screen size with optimum tilt position. For the proposed municipal waste treatment plant, we’re procuring heavy-duty trommel screen from Krause Manufacturing, Inc. To suit the need of the system, a screen with specifications, as mentioned in table 5, is selected.Table 5: Trommel screen specificationsEquipment DetailsLength 15 mDiameter 2.4 mThroughput per hour35 tonsDrum inclination4o(Krause Manufacturing, Inc., 2012)Figure 5: An installation of trommel screen by Krause Manufacturing, Inc.(Krause Manufacturing, Inc., 2012)A screen size of 70 mm is used with deflectors to increase the overall efficiency of the trommel. When garden waste is being pre-treated, separated smaller fractions can be taken directly to compost after removal of lighter fractions in air classification process. However, for bio-waste, it is necessary to subject the smaller fractions to fine shredding as per system requirements of the fermenter.2.2.4 Density SeparatorFollowing screening where the particle is separated based on size, a final separation equipment is incorporated which segregates the waste into two fractions based on particle density. This helps in removing any plastic, film, paper or other lighter unwanted materials that may have passed through up-stream pre-treatment processes. A density separator utilizes air stream for material separation. Smaller and or denser materials are segregated by air and drop down onto the discharge conveyor. Since air is being blown to perform separation, emissions of foul odor is a disadvantage. This is minimized by circulating most of the air stream. Another possible reduction in efficiency can be observed when bio-waste with high moisture content is passing through the density separator. Thus, a very robust system is chosen which shall be designed, engineered and supplied by our sub-vendor EMS Turnkey Waste Recycling Solutions to segregate all kinds of municipal waste, including bio-waste. Figure 6: Schematic of Density Separator(EMS Turnkey Waste Recycling Solutions, 2018)The model DS 3500 is selected taking into consideration the system requirements of the plant and to match the throughput of up-stream equipment. The specifications are as mentioned in table 6.Table 6: Density Separator SpecificationsEquipment DetailsLength 6.09 mWidth 1.51 mHeight7.82 mThroughput per hour35 tons(EMS Turnkey Waste Recycling Solutions, 2018)2.2.5 ShredderThe last unit operation included in the proposed waste management facility is size reduction. A fine shredder is used for this process. Impurities are eliminated before feeding waste stream into the shredder and it is vital in particular to remove metals from the waste stream owing to the fact that they have the potential to disrupt the operation of shredder by clogging the cutting blades. This may also lead to complete shut-down of the equipment and subsequently bring the entire process to a halt. Shredding of sorted waste is an important stage here because of the requirement of downstream process of dry fermentation wherein particle size is warranted to be less than 40 mm. It shall also facilitate enhanced microbial activity by shredding the waste results into increase in surface area of the particles. Thus, we shall procure a shredder from our sub-vendor Weima Maschinenbau GmbH which can carry out fine shredding of municipal waste of any sort. It is made up of knife-like blades, one rotating clockwise and the other anti-clockwise and in between the two, particles are shredded and fall underneath the blades through a screen onto a belt conveyor. The model selected is Weima Powerline and it is capable of shredding the waste down to 30 mm particle size. The specifications are as depicted in table 7.Table 7: Shredder SpecificationsEquipment DetailsLength 5.25 mwidth 2.0 mHeight4.39 mRotor knives80 x 80 mmRotor diameter800 mmThroughput per hour35 tonsFigure 6: Installed WEIMA Powerline shredders MBT plant in Waterbeach, England (Weima Maschinenbau GmbH, 2017)