9 Transfer station

Dr. Yogalakshmi K. N

epgp books

 

Objectives:

 

1.  To understand the concept and significance of transfer station

 

2.  To study in detail about the benefits associated with the use of transfer stations

 

3. To understand the design issues associated with transfer stations

 

4. To gain knowledge on the transfer station technologies and the problems associated with transfer station

 

1.0 Introduction

 

Transfer station is a centralized facility located between the collection and disposal site. It is also called as an intermediate station where waste from smaller collection vehicles are unloaded and re-loaded back into large vehicles for transport to a disposal or processing site. It is built in between the source of generation and disposal site. In addition to loading and unloading of waste, transfer stations are also used as storage and waste processing sites. They have facilities for waste separation/ segregation, size and volume reduction (shredding, compaction) and component separation. In some cases, they also have waste treatment facilities such as incineration, pyrolysis and composting. In areas with narrow and congested lanes where use of compacted trucks is limited, the transfer stations are used as a facility to transfer the waste from small vehicles and non-compacted trucks for segregation reloading into large vehicles. Likewise, when waste is not dense, they are brought to transfer station and compacted. The transfer station also serves best when the distance between the collection zone and disposal site is very high. The transfer station also serves as a garage for temporary parking and vehicles servicing. The major limitation of transfer station includes additional construction for building transfer stations and labour cost. It also consumes high amount of energy for waste transfer, segregation and processing. The problems associated with transfer station is that it attracts flies and other insect vectors and creates odours. Traffic and noise due to small and large collection vehicles, collectors, drivers, etc., invite the resentment of the communities living in the vicinity of transfer stations

 

The need for transfer station is based on the following criteria

 

Type of waste received: Co-mingled waste requires an intermediate facility, transfer station for segregation and sorting of waste before disposal. Likewise, the amount of waste generation also destines the need for transfer station. Considerable amount of waste essentially requires transfer station.

 

Process involved in recovering materials: waste that has been already segregated does not require a transfer station. However, co-mingled waste that contain recyclables are to be segregated before been transported to respected industries. The segregation, be in manual or mechanical is done and the materials are recovered at the transfer station. Example, ferrous and non-ferrous waste present in comingled waste is separated.

 

Types of collection vehicle: The collection vehicles are chosen depending upon the depending characteristic of the area. Areas with narrow and congested lanes require small vehicles for waste collection. However, areas with wide lanes can utilize compacted trucks as collection vehicles. As mentioned earlier, the small collection vehicles require transfer station for unloading the waste and reloading them to bigger collection vehicles.

 

Site topography and access

 

2.0 Types of transfer station

 

Based on the size, the transfer station are classified into three types

 

Small transfer stations: small transfer stations can hold waste up to 100 tonnes per day. It is a direct discharge station and does not have intermediate waste storage area. Depending upon the site characteristics and weather conditions the transfer station can be Indore or outdoor. Small transfer stations focus only on segregation and hence does not have processing and treatment facilities ( size and volume reduction units). Population, population density and frequency of collection determines the size of transfer station.

 

Medium transfer station: It is similar to small transfer station except for the capacity of waste it can hold in a day. The capacity of medium transfer station is about 100 to 500 tonnes per day. They can also be constructed indoor or outdoor. In some cases few processing units are made available in medium transfer stations.

 

Large transfer station: Such station is designed especially for municipal collection vehicles. Sometimes private companies hold these large transfer stations for heavy commercial use. Mostly the large transfer stations are constructed outdoors. In addition to segregation, component separation size and volume reduction is carried out in the large transfer station. Few large transfer stations have inbuilt incinerators and composting facilities. Before the vehicle enters into the large transfer stations, it is weighed and taken to an area provide with a pit, platform and a waiting trailer. The waste is unloaded and taken through the trailer for segregation and processing.

 

3.0 Design of large transfer station

 

Depending upon the type of vehicles and distance between the source of generation and disposal site, the design of a large transfer station fall into three categories: direct discharge non compaction station; platform/pit non compaction station and compaction station.

 

a) Direct discharge non compact station: In this design the waste brought by the small collection vehicles are discharged into a transfer trailer. The station consists of two main operating floors. One on the top floor where the waste from collection vehicles is dropped through a hopper into an open top trailer on the lower floor. The trailers are provided with special scales which on receiving maximum load stops the dumping process. The waste on the trailer is covered with tarpaulin to avoid the entry of water into the waste. The waste in the trailer is distributed using a stationary crane provided with a bucket.

 

b) Platform/pit non compaction station: In platform type transfer station the waste is dumped on to a platform or a pit using a crane. The platform or pit serves as temporary waste storage site. The recyclables or any unacceptable materials are segregated by hand picking, while waste is stored in the platform. Through front end loaders the waste is pushed into open top trailers. Similar to direct discharge station platform has two levels and pit has three levels. The major disadvantage of this facility is increased construction cost contributed by increased floor space. This facility can manage huge amount of waste and act as temporary storage site.

 

c) Compaction station: This station is favored when waste compaction is necessary. Mechanical compactors are installed in this transfer station to compress and bale the waste. Hydraulically powered compacters are commonly used in this station. The waste is fed through a chute either directly from collection trucks or from pits. Compaction stations are mostly preferred during the following conditions: Baling of waste for shipment; the site location does not permit multilevel-building; due to size restrictions when open trailers cannot be used.

 

The compaction facilities suffer from various disadvantages: efficiency of compactor drops wit time, non-availability of spares parts and labor, improper maintenance of the equipment.

 

4.0 Problems of transfer station

 

The problems associated with transfer station include:

 

  • Site selection: Procurement of a suitable site is very important. The site should be isolated from other residential area to avoid problems of odour, flies and insects. The noise caused by the vehicles create disturbance to the public. Hence, it always better to have the transfer station in an area where the effect of nuisance will be minimum.
  • The transfer station will attract flies and other insect vectors which might lead to diseases among the waste handlers and public.
  • Since waste is stored for more than a day in transfer station generate foul smell and bad odour due to the degradation of the waste.
  • Dust is again a major problem in transfer station. The source of dust can be unpaved roads or wind or waste material reaching transfer station.
  • Traffic congestion can occur due to vehicles reaching the transfer station which again leads to public nuisance.
  • The level of noise pollution is also high in the vicinity of transfer station.
  • The size of the transfer station is planned based on the following factors
  • The quantity of waste generated within the service area The rate at which the waste is delivered to the station
  • Changes in population growth, segregation pattern and recycling programs Peak flows of waste should also be considered.
  • Types of vehicles delivering waste
  • The types of waste brought to transfer station eg. Municipal solid waste, construction waste, e waste, compacted waste or loose waste etc.
  • Influence of seasons in waste generation pattern should also be considered Time period during which waste is brought to the transfer station
  • Facilities available in the transfer station eg. Availability and working conditions of trailers, railcars, barges. The speed of loading and unloading of waste materials in transfer station is also considered.
  • Expected increase in the quality of waste during the life of the facility.
  • Potential to upgrade the facility to hold other facilities such as recycling facilities, and waste-to-energy facilities.

 

5.0 Capacity of transfer station

 

Capacity is one important factor that needs to be considered while constructing the transfer station. It should be designed such that they have a storage capacity of two days. The transfer station is planed/designed to have adequate space for additional waste storage. Space should also be available for upgrading the transfer station with waste processing equipments. Adequate space should also be provided for waste unloading, vehicle and equipment maneuvering. Design should have provision for weighing bridge office and other related facilities. The factors considered while designing the transfer station include:

  • Type of collection vehicles
  • Capacity of collection vehicles
  • Number of collection vehicles to used the facilities
  • Waste storage period on tipping floor
  • Time taken by collection vehicles to unload the waste
  • Time of arrival and departure of collection vehicles
  •   Waste sorting and processing facility to be setup in the station
  • Capacity of transfer trailer
  • Time requirement for stable operation, attachment and detachment of trailers from tractors and compactors

 

The capacity of transfer stations is determined based on the following methods.

 

i.Pit stations:

 

Based on the rate at which wastes can be unloaded from collection vehicles: C = Pc x (L/W) x (60 x Hw/Tc) x F

 

Based on rate at which transfer trailers are loaded:

 

C = (Pt x N x 60 x H t)/(Tt + B)

 

ii.    Direct dump stations:

 

C = (Nn x Pt x F x 60 x Hw)/ [((Pt/Pc) x (W/Ln)) x Tc + B] iii. Hopper compaction stations:

 

C = (Nn x Pt x F x 60 x Hw)/[(Pt/Pc x Tc) + B]

 

iv.    Push pit compaction station: C = (Np x Pt x F x 60 x Hw)/[(Pt/Pc x W/Lp x Tc) + Bc + B]

 

Where:

 

C = Station capacity (tonnes/day)

 

Pc = Collection vehicle payload (tonnes)

 

L = Total length of dumping space (feet)

 

Hw = Hours per day that waste is delivered

 

Tc = Time (in minutes) to unload each collection vehicle

 

F = Peaking factor (ratio of the number of collection vehicles received during an average 30-minute period to the number received during a peak 30-minute period) Lp = Length of push pit (feet)

 

Np = Number of push pits

 

Bc = Total cycle time for clearing each push pit and compacting waste into trailer Pt = Transfer trailer payload (tonnes)

 

N = Number of transfer trailers loading simultaneously

 

Ht = Hours per day used to load trailers (minutes)

 

B = Time to remove and replace each loaded trailer (minutes)

 

Tt = Time to load each transfer trailer (minutes)

 

Nn = Number of hoppers

 

Ln = Length of each hopper(feet)

 

6.0 Transfer Station Technologies

 

The transfer technology describes the method by which the waste is handled within the transfer station. Beginning from the waste unloading from the collection vehicle on to the tipping floor to waste processing and until it leaves the transfer station is taken care off through different technologies. The commonly used technologies are given below.

 

6.1 Open top transfer trailers: It is a simple and flexible technology which does not use any compactors or bailing units. In the open transfer trailers, the waste is unloaded directly into the “open top” of the trailer. Mostly the waste is unloaded on the tipping floor to facilitate inspection and segregation of materials before being dropped into the trailer. The trailers are more than 100 cubic yards to get a good payload. The waste dropped in the trailer is not compacted. This technology is preferred for low volume operations.

 

6.2 Surge Pit: The surge pit technology is used as an intermediate step in open-top or precompactor systems. This technology reduces the number of trailers as they serve as a storage unit for peak flow of waste. The waste is compacted by bulldozer before loading. The major disadvantage of this system is the material recovery or sorting is not possible.

 

Source: https://www.epa.gov/sites/production/files/2016-03/documents/r02002.pdf

Figure 2 Surge Pit

 

6.3 Stationary compactors: The waste is unloaded onto the tipping floor and compacted using a hydraulic ram in the transfer trailer. The trailer should be strong enough to withstand the load of compacter ram and waste material. It is usually made of reinforced steel. This technology is not so popular among other technologies.

Source: https://www.epa.gov/sites/production/files/2016-03/documents/r02002.pdf

Figure 3 Compactor system

 

6.4 Precompactor system: In this technology the waste is compacted before being discharged into the trailer system. A cylinder with hydraulic ram is used to compact the waste as dense log of waste. The log is then pushed to the trailer. The log moves along the trailer by gravity and this technology is termed as “walking floor” technology. The investment cost of the precompactors are very high.

 

6.5 Balers: Balers are equipment generally used to compress and compact the waste. The waste is compacted into dense bales which are tied by wire strap to hold them intact. Paper and metal are compacted or baled by this method. Balers are generally used in high volume operations and they require special assistance or equipment to place them in the landfills. Similar to precompactors, balers too involve high capital costs.

 

 

6.6 Intermodal container system: Waste brought in the collection vehicle is dropped on the tipping floor. The waste from the tipping floor is loaded into intermodal containers. The loaded containers are placed in flatbed trailers and railroad flatcars to ease the transportation of waste to landfills. The containers are sealed and stored onsite for more than 24 hours until all the containers are filled. The intermodal containers control both moisture and odor. The containers are unloaded by tippers in landfills. It is one of the cost effective option and they also reduce traffic congestion.

 

7.0 Benefits of transfer station

 

The benefits of the transfer station are as follows.

  • minimizes the cost of collection
  • minimizes the fuel cost of vehicles and labour
  • recovery of recyclables
  • increased flexibility
  • Reduces the number of collection vehicles through large trailers
  • Efficient operation of collection vehicles and rapid return to work
  • We can choose a disposal site far away from population areas
  • Opportunity to inspect and process the waste

 

8.0 Summary

  • To summarize, in this module we have familiarized about
  • Transfer station and their significance in waste collection Types of transfer station
  • Design of transfer station
  • determination of transfer station capacity problems of transfer station
  • Different technologies used in transfer station benefits of transfer station

 

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References