WO1996031441A1

WO1996031441A1 - Combined waste treatment

Info

Publication number: WO1996031441A1
Application number: PCT/AU1996/000199
Authority: WO
Inventors: Paul Oakes
Original assignee: A.C. Tipping Pty. Ltd.
Priority date: 1995-04-06
Filing date: 1996-04-04
Publication date: 1996-10-10
Also published as: AUPN221195A0

Abstract

An integrated waste treatment system for the combined processing of sewage sludge and municipal solid waste in which primary (raw) sewage sludge is utilised. The combined system allows for the sewage sludge to be used as a feedstock when composted with the solid waste. The high nitrogenous content and biological activity of the raw sludge ensures effective composting. Separation of the sludge feedstock into a high density pumpable sludge stream and a low density stream prior to co-composting is required, with the low density stream being at a centrate level suitable for immediate use or further treatment as grey water.

Description

COMBINED WASTE TREATMENT Technical Field

The present invention relates to an improved system for integrated sewage treatment and waste composting, and particularly to the production and use of primary (raw) sewage sludge in this process. Background Art

Treatment and disposal of household effluent or municipal sewage is one of the environmental necessities of urban life. The vast amounts of sewage that is daily generated and flows through the sewers, is required to be quickly and effectively treated. Ideally, it is converted to environmentally friendly, manageable materials for subsequent use or disposal. Given the large quantity of material which is required to be handled, capital and cost efficient processing is desirable. Similarly, solid garbage such as household refuse and factory waste, collectively known as municipal solid waste (MSW), requires efficient treatment to minimise the requirement for transport and disposal in landfills or incinerators.

Conventionally, treatment of municipal sewage results in the production of municipal sewage sludge (MSS), which may be treated in various ways, or disposed of in landfill sites. However, as the treated MSS generally has a high heavy metal content, its simple disposal as landfill is environmentally problematic. US5047349 to Bedminster Bioconversion Corporation Inc. discloses a means by which MSS is treated or sterilised and dewatered and then utilised in a process for the treatment of Municipal Solid Waste (MSW). In such a system the treated MSS and MSW, by co-mingling, produce a compostable matrix, the treated MSS being used as a feedstock to compost the garbage. Alternatively, as disclosed in US Patent No. 3 847 803 to Fisk, the combined MSW and MSS are both treated prior to composting.

As the means for composting solid waste (MSW) and treating liquid sewage conventionally entail totally disparate treatment processes, different specialist technologies, and are often performed by different municipal bodies, respective treatment sites for each have naturally been established independently of each other. Thus, this current means of disposing of the MSS as a feedstock, necessitates treating and dewatering the sewage sludge and transporting said dried sterilised sewage sludge from the sewage treatment site to the site of MSW composting. This disposal of treated MSS as a feedstock is therefore relatively expensive to effect. Further, because the sterilised MSS is transported in said dried or solid form, before it is used as the feedstock in the MSW composting system, it is necessary to re-suspend it, generally using treated, municipal water supplies.

Thus, it is an object of this invention to provide an integrated waste treatment system of simplified design. Summary of Invention

Accordingly, the present invention provides an integrated system for processing a raw sewage sludge feedstock and a municipal solid waste feedstock, comprising the steps of ; separating the raw sludge feedstock into a relatively high density, pumpable sludge stream, and a lower density stream; and co-composting said pumpable sludge stream and solid waste feedstock.

Preferably, the raw sewage is passed through a sedimentation tank or the like, prior to separation into higher and lower density streams. Preferably, the lower density stream is at a centrate level suitable for use or further treatment as grey water.

Preferably, the separation is performed by a mechanical system, such as a centrifuge or the like, more preferably a vertical centrifuge.

It is also preferred that a polymeric flocculent be mixed with the raw sewage sludge during the separation to obtain maximum results. The composting process is suitably the Bedminster Bioconversion

Corporation Inc. process described above.

By providing the system arrangement described above, it is now possible to avoid treating the raw sewage sludge feedstock before it is added to the MSW feedstock to be composted. The addition of the sludge to the MSW dilutes the relatively high heavy metal content of the sludge in the end product, thereby removing the problem of disposing of a high heavy metal content sludge. More advantageously, as the steps of treating, dewatering and transporting the dried, treated sludge are now superfluous, the time and labour costs associated with these operations are no longer required to be outlaid, and the capital requirement of the plant is reduced.

Further, as the raw sewage sludge is not treated and dried, but is pumped in liquid form straight to composting, the step of re-suspending the sludge before composting, is redundant in the above described invention. Thus, the addition of large quantities of potable water to the dried sewage sludge feedstock as occurs in the prior art, is not necessary under this invention and wastage of a valuable resource is thereby prevented when using this system. Another advantage of the above combined system is that a variety of different agricultural reuse products are produced. These include dried pelletised sludge, high quality enhanced pelletised sludge and compost. A further advantage is the fact that the untreated sludge provides an excellent source of Nitrogen and biological activity. In conventional sludge processes, the biological activity is deliberately substantially removed by sterilisation, and the nitrogen content reduced. According to the present invention, the high nitrogen and biological activity are in fact advantageous in the co-composting of MSW, and so the former sludge treatment processes are not required. The invention thus provides an integrated waste treatment system having a cohesive structure resulting in refined, time efficient processing of municipal sewage sludge without the need for any extraneous and costly operational steps. Brief Description of Drawings

An illustrative embodiment of the present invention will now be described with reference to the accompanying figure, in which : Figure 1 is a schematic block diagram showing a typical system according to the present invention. Description

A preferred embodiment of the invention comprises an integrated sewage and MSW treatment plant in which the primary or raw municipal sewage consisting of a carrying liquid and semi-solid particles, passes through a grit removal tank and primary screening unit, and is pumped into a primary sedimentation tank whereby the more dense phase being untreated municipal sewage sludge or slurry (MSS) settles at the bottom of the tank, while the clear liquid is discharged as a readily treatable grey water stream. The sedimentation tank may be of conventional design, or any other suitable mechanical system may be used.

The untreated MSS is then pumped into a separator, which may preferably be a centrifuge. This effects a partial compacting of the sludge and results in its separation into two, still untreated, centrate streams.

The untreated low density centrate stream known as 'grey water¹ ideally consists of up to 0.05% solids and can be readily treated for release into the environment, and is suitable to be pumped as direct discharge into the ocean.

The untreated high density centrate stream consists of up to 13% solid waste which is pumpable as liquid through pipes to a composting digester as disclosed in US5047349 to Bedminster Bioconversion Corp Inc. Said digester is in the form of a rotating compartmentalised compost drum where said high density centrate stream is mixed with MSW and aerobically fermented. The fermentation process is staged such that the MSW and MSS feedstocks are treated with successive groups of aerobic micro-organisms.

A particular embodiment of the instant invention is a project involving the construction and operation of a facility processing combined waste streams of both sewage sludge and municipal solid waste (MSW) producing compost and pelletised sludge for agricultural reuse.

The sewage sludge which is sourced from various sewerage plants (approx. 30,310 dry tonnes/year) is pumped from the plants to the central processing facility via pipelines. The MSW (approx. 150,000 tonnes/year) is obtained from local councils and transported to the central facility.

The MSW is combined with part of the sewage sludge and digested and composted in a Bedminster process and sold into the agricultural market.

The quantity of the sewage sludge not currently used in the Bedminster process (approx. 21 ,000 dry tonnes p. a. out of 30,310) is dewatered in a centrifuge plant and then dried and pelletised using an indirect drying process using steam as the heating medium. This unused amount of sewage sludge however will decrease when further quantities of MSW are obtained because co-composting will be able to occur on a larger scale, using large quantities of sewage sludge.

The Bedminster composting process consists of the following elements: 1. Presorting to manually remove bulk non-compostable objects.

2. Loading into digesters of MSW and sewage sludge.

3. Primary screening and separation of inorganics.

4. Digestion over a 3 day period.

5. Aeration over a 28 day period. 6. Secondary screening of inorganics. 7. Compost storage and despatch.

The particular preferred features of this embodiment are described in detail below.

1 .1 Sewage Sludge Transport 1.1.1 Sludge Particle Size Distribution

Raw sewage sludge contains a significant quantity of coarse organic particles which have a low specific gravity, together with a percentage of inorganic matter, typically sand. The organic material is of little significance in slurry pipeline design. In a test performed in 1990, a sludge sample was washed to remove the light material, and a screen analysis performed on the heavier material. The particle size of this material is presented in Table 1.1.1.

TABLE 1.1.1 PARTICLE SIZE OF SLUDGE RESIDUE

Screen Size Percentage Retained Comment (micron) (% wt)

1400 39.75 Large seeds & shells

300 34.32 Seeds & shells

90 23.46 Sand (white & black)

-90 2.47 Sand (white & black)

The significant material for pipeline transport are the fractions smaller than 300 micron, which have a specific gravity of approximately 2.5, and a relatively high settling velocity. Seeds are organic, with a specific gravity near that of water, and shells have a relatively large projected area to their mass, and transport relatively easily when in low concentration in a pipeline.

The particulate material represented approximately 18% of the solids in the sample tested. The inorganic matter represented 4-6% of the total sludge solids.

1.1.2 Storage Tanks and Pumping Systems

The MSS is delivered to the central processing facility via pipeline from various storage tanks at sewage plants. Prior to delivery, the sludge is preferably processed to remove rags, grit and other large particles and solid matter. Preferably it has also passed through a sedimentation process.

During pipeline operation the pipeline is operated at a flow which is at least the minimum flow rate, and the sludge concentration is determined by the sludge mass continuously delivered to the storage tanks, diluted by the makeup water introduced to maintain the level. 1.1.3. Central Processing Facility

The pipeline at the central processing facility terminates in a scraper receiver.

The incoming pipeline is equipped with flow, density and pressure instrumentation. An agitated storage tank is required at each sewage plant pump station to provide surge capacity for the pipeline, and to allow some homogenisation of the sludge being fed to the pipeline. Agitated storage is also required at the central processing plant. 1.2 Sludge Dewatering and Drying Plant 1.2.1 General

Sludge pumped to the central processing facility is stored in a sludge storage tank. The sludge plant comprises a sludge dewatering plant, a controls area, and a sludge drying plant. The sludge dewatering plant provides feed to both the sludge drying plant and the co-composting Bedminster plant. 1.2.2 Dewatering Plant 1.2.2.1 Design Basis

The dewatering plant is designed to dewater 30,000 tonnes per year of raw sludge @ 4% dry solids content into two process feed streams. A solids content of 22-24% is required for the sludge drying process and separate process stream of 12-15% solids required for the Bedminster process. The drying plant uses a Stord drying process.

The key design loading parameters for the sludge dewatering facility is shown in Table 1.2.2. TABLE 1.2.2

Peak Capacity Bedminster Stord Drying Process

Volume Solids Volume Solids Volume Solids

90m3/hr 4% 24m3/hr 12-15% 90m3/ r 22-24%

Both Process Streams are fed from a common storage balance tank with dedicated feed pumps to each centrifuge.

As the Bedminster process is a batch process the feed stream for the sludge drying process is capable of handling the full 90m3/hr flow. 1.2.2.2 Dewatering Plant

Sludge from the storage tank is fed via individual pumps to each centrifuge.

Chemical dosing of the sludge feed lines prior to feeding to the centrifuges is provided by a polymer dosing system located inside the dewatering plant. Chemical conditioning of the sludge improves the solids capture and centrate quality from the centrifuge. There are 4 Alfa-Laval Model NX4565 Decanter centrifuges in the dewatering plant. The equipment is arranged so that only three centrifuges are operating with one standby at any one time. When the Bedminster Plant is operating, one centrifuge provides the feed at 24 m3/hr and two centrifuges feed the drying plant at 33 m3/hr each. When the Bedminster Plant is off, 3 centrifuges feed the drying plant at 30 m3/hr each.

The Alfa-Laval Decanter centrifuge uses a rotating cylindrical bowl to generate separation between solid and liquid materials in the sludge. The solids are sedimented against the sides of the rotating bowl wall, and is forwarded to discharge openings by an axial rotating screw conveyor. The difference in density between the solids and the liquid, allows the less dense liquid to form a concentric inner layer which overflows through adjustable weirs at the end of the bowl. The concentrated sludge leaves the centrifuge by gravity discharge, to a transfer system, while the centrate is removed by gravity discharge to collecting tanks and then to a centrate sump.

The centrate from sludge dewatering contains suspended non-settling solids. It is proposed to treat the centrate by a small treatment plant. Alternatively, the centrate can be discharged to sewer.

Dewatered sludge for the Bedminster system is transferred to a sump, where it will be pumped to the Bedminster process. The dewatered sludge for the Stord drying process is transported to the drying building via a belt type conveyor. This conveyor links with a reversible feed conveyor and is used to feed the two dewatered sludge storage silos in the sludge drying plant. 1.3 MSW Bedminster Plant 1.3 General

The Bedminster system is a process for stabilising municipal biosolids and municipal solid waste by co-composting. Municipal solid wastes are sorted and screened so that the biodegradable portion, including paper stock products, green waste, and food waste is composted with the sewage sludge. The non- biodegradable portion of municipal solid waste is recovered as recyclable materials.

The plant is designed to process the following annual quantities of waste: • Municipal Solid Waste (MSW) 150,000 tonnes/year

• Municipal Sewerage Sludge (MSS) 9,000 tonnes (dry)/year 1.3.2 Design Basis

The Bedminster plant is designed to process the following material daily rates: • Municipal Solid Waste (MSW) (475 tonnes/day)

• Municipal Sewerage Sludge (MSS) (238 tonnes/day [12% solids]) 1.3.3 Process Description 1.3.3.1 Raw Material Receipt

The MSW is delivered to the plant by truck. MSS is delivered to the plant by direct pipeline from 12% solids sludge holding tank located at sludge drying plant.

Trucks deliver MSW into the tipping building, dumping the MSW on the elevated floor. A front end loader (one working each side of the building) moves the material to the digester feed hoppers. Material is manually sorted and separated on the tipping floor. 1.3.3.2 Digesters

Municipal solid waste and wastewater sludge is fed into a digester. The digester is a compartmentalised, rotating steel cylinder. The digester introduces the wastes to microbial activity and provide an optimal environment for composting. The feed in the digester is subjected to 3 days of high rate composting. The digester reduces the particle size of materials, homogenise the solid and liquid wastes, and separates non-compostable materials from compostable materials by biological and mechanical processes. The rotation of the digester breaks open bags, and continually macerates the material. These actions reduce the particle size of the feed, exposing more sites for biological interaction. The rotation action also maintains an aerobic system by exposing the feed to air blowing through the digester. The speed at which the digester rotates can influence the degree of degradation occurring in the digester. The speed of rotation is adjusted automatically depending on the amount of feed in the digester, the normal operating speed of rotation is 80 hertz. The end discharge from the digester is a rough compost which is transferred to primary trommel screens via conveyors.

The MSS will be stored in a tank located at the sludge drying facility. Other liquid waste streams are collected in the plant at various lift stations and pumped to the holding tank. The liquid waste is added to the digesters along with the MSW and MSS during the loading period.

The waste streams are introduced to the five digesters during a sixteen hour loading cycle. The MSW is fed into the digester by a hydraulically operated ram feed assembly. The MSS and liquid waste streams are pumped into the digester at the same time as the solid waste streams. The digesters are divided into three compartments. The material takes three days to travel through the three compartments. Five Eweson Digesters 4.87 m diameter x 73 m long shall be provided.

The digesters are Bedminster custom designed units fabricated by ADI, containing three compartments with transfer mechanisms. Each digester is capable of processing 95 tonnes of MSW per day. Each digester is equipped with a 298 kW electric motor with variable speed drive. 1.3.3.3 Initial Screening

The digested material is sent to the primary trommel screen by conveyor. The material is screened, separating the organic and inorganic fractions. The inorganic materials are loaded into open top transfer trailers for disposal at the landfill, the organic materials are transferred by tripper conveyor to the aeration building for further processing.

The rotary trommel screens separate the larger fraction of the rough compost stream consisting mainly of inorganic materials, inert and undegraded by the digesters from the smaller, mainly organic portion of the waste stream. The trommel is a cylindrical steel deck with perforations suitably sized (approx 3 cm or 1 1/₄ in.) so that smaller material can drop through the structure. Most of the organic material would have been degraded by the digester to pass through the trommel perforations. The rough compost from the digesters are fed into the sloped feed end of the rotating trommel. The feed falling through the trommel is collected by a conveyor for further processing. The portion of the feed that does not drop through the perforations is discharged from the lower end of the trommel, and collected onto conveyors leading to further processing for metals separation.

Two primary trommel screens are included, rated at 45 tonnes per hour. These units are standard industrial design with screen modifications. The screens are designed to separate materials above/below 32 mm diameter. The complete assembly is built on a structural steel frame which is supported at grade level. Also, walkways are part of the framework which allow access around each unit. 1.3.3.4 Aeration

There are four identical aeration bays within the aeration building which is 101m by 155m. These chambers are approximately 25 metres x 146 metres long. Here the rough compost material is processed for an additional 28 days. Under the floor air channels allow cooling air to circulate through the compost beds controlling the temperature of the process. The fans moving the cooling air are controlled by the PLC -systems. Each aeration bay is equipped with a bridge crane mounted automated turning machine. This turning machine moves the compost from the feed end to the discharge end of the building.

The conveyor from the trommel containing the organic compost deposits the material to a concrete aeration floor within an enclosed building. Further composting takes place on the aeration floor using a combination of aerated static pile and turned windrow methods.

Elongated piles approximately 2 metres high are constructed over aeration pipes. The piles are periodically turned by an automatic turning machine. The heat generated in the composting process destroys pathogens found in the wastewater sludge. The windrow method requires subjecting the composting material to a temperature of 55° Celsius for 15 consecutive days with a minimum of 5 turnings, to achieve pathogen destruction. The temperature required for the aerated static pile is 55° Celsius for at least 3 consecutive days.

Temperatures in the piles are monitored by probes which are lowered into the piles. The material is detained for 4 weeks on the aeration floor to allow for sufficient degradation.

The turner mechanism is suspended from a travelling bridge crane which moves on a rail system. The machine begins operation at the discharge end of the operation floor and moves sideways through the pile. The mechanism lifts and aerates the compost whilst moving the material forward towards the discharge end of the aeration floor. The turning machine returns to the extreme side of the aeration floor after turning the entire length of the aeration floor. The cured compost is collected in a hopper at the discharge end of the aeration floor. Four Bedminster designed fabricated units are provided. These units are designed to agitate, aerate, mix and move up to 7,182 cubic metres per day. These units are supported by a movable bridge crane structure that spans approximately 24 m. These machines are electrically powered and are controlled by the PLC system. 1.3.3.5 Final Screening

The composted material is transferred to the final screening area by front end loader to the final screen conveyor hopper. There are two final screen assemblies included. After screening, the material is sent to the compost storage area for ten days of additional curing time. This is also transferred by front end loader.

The final screening area is located at the end of the compost storage area. The final screens are fed by a conveyor running from a centrally located hopper at the ends of the aeration bays. Metals are removed from the discharge stream containing large inorganic materials from the trommel. A pulley magnetic separator installed overhead of the discharge stream removes ferrous materials by magnetically lifting off materials from the conveyor. Ferrous metals removed from the separator is dropped into a side discharge chute into a storage container. Non-ferrous metals are removed by an eddy current separator which generates a strong electric field, so that it can repel non-ferrous metals towards an alternative stream, creating a flow split from the normal flow of the residual stream. The electric field of the separator is generated by a rare earth magnetic element, rotating at high speeds inside a slowly revolving shell. Final screening is performed by a trommel screen and stoners to remove clumps and inert inorganic material from the aeration floor compost. The major components of the final screen include the conveyor from the hopper, a trommel, stoner(s), a cyclone dust collector, and a gutter cleaner. The trommel has a series of perforation sizes. The material falling through the larger perforations are passed to a stoner to remove glass and clumps. The reject material separated from the stoner requires disposal. The remaining compost from the stoner, and the compost falling through the smaller sized perforations of the trommel is conveyed to the compost storage area. Compost from the final screen also provide the compost culture used as an inoculant in the digester to help the raw municipal solid waste and sludge start composting.

Two final screen assemblies are included, each rate at 23 tonnes per hour. These units are standard material design with screen plate modifications. The complete elevated assembly is built on a structural steel frame which is supported at grade level. Also walkways are part of the framework which allow access around each unit. The screens have three different perforations, 4mm, 6mm, 9.5mm diameter. 1.4 Odour Control

Odour control for the process area is provided by soil bed biofilters. Total Biofilter loading is 125 nrι3/second. Detention time in the bed will be a minimum of 50 seconds.

The odour control unit is made up of a number of identical units. Capacity is selected to ensure that, at any time, one unit can be shut down for soil refurbishment and bed maintenance. The odour control unit consists of a layer of soil (nominally 1m deep). This overlays the pipe distribution network which is situated in a gravel base to enable even distribution of foul air through the soil. The bed itself consists of a layer of soil materials. Foul air is drawn into the soil bed by a centrifugal fan located near the soil bed unit. The foul air is directed to the soil bed distribution network via an above-ground manifold (FRP). The pipework which distributes the air below the soil bed consists of standard sub-soil PVC drainage pipes. The ductwork shall be designed for a maximum normal working pressure of 2kPa. Over-pressures of up to 2.5 kPa may be expected. Two Foul Air Fans are provided. Each is designed for 8 rrι3/s at 4000 Pa pressure.

The soil bed is designed for replacement approximately every five years, however this is dependent on operational experience. Example Untreated raw sewage was passed through a grit removal tank to remove grit, through a primary screening unit to remove large items, and then into a primary sedimentation/clarification tank. The sewage sludge feedstock was taken from the bottom of this tank, said sewage sludge having a percentage of solids in the range of 3%. The sludge was pumped by mono pump through two lengths of hose to a P600 decanter/separator where it was centrifuged for typically 10-25 minutes. The cake (or high density stream) consisting of approximately 10% sludge solids content was discharged from the decanter into a drum and later into the plant's grit removal tank, while the centrate was discharged directly into the grit removal tank.

Samples were taken simultaneously of the centrate, cake and influent flows. Centrate and influent sample size were typically in the 500ml to 1 L range, cake samples were typically in the range of 100-300mg.

The cake or sewage sludge feedstock may be then pumped into the composting digester drum as disclosed in US5047349 to Bedminster Bioconversion Corp Inc. There it is mixed with the MSW and treated with successive groups of aerobic bacteria maintained at optimum conditions to produce efficient and thorough aerobic decomposition of the organic materials. It will be appreciated that the described implementation is only by way of example, and other implementations and additions are possible within the broad inventive concept.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A waste treatment system for processing a raw sewage sludge feedstock and a solid waste feedstock, comprising the steps of: a) separating the sewage sludge feedstock into a relatively high density pumpable sludge stream and a lower density stream; and b) co-composting said pumpable sludge stream and said solid waste feedstock.

2. A waste treatment system according to claim 1 wherein the separation is performed by a mechanical system.

3. A waste treatment system according to claim 1 or claim 2 wherein the separation is performed by a centrifuge.

4. A waste treatment system according to any one of the preceding claims wherein said lower density stream has a centrate level of up to 0-05% solids.

5. A waste treatment system according to any one of claims 1-3 wherein said high density sludge stream consists of up to 15% solids.

6. A waste treatment system according to any one of claims 1 -5 wherein said lower density stream requires no further treatment before re-use.

7. A waste treatment system according to any one of claims 1-6 wherein a polymeric flocculent is mixed with said raw sewage sludge during said separation into lower and higher density streams.

8. A waste treatment system according to any one of claims 1-7 wherein said raw sewage sludge feedstock results from the application of a sedimentation process to raw sewage.

9. A waste treatment system according to any one of claims 1-7 wherein said raw sewage sludge feedstock results from the application of a physical screening process to raw sewage.

10. A method for processing a raw sewage sludge feedstock and a solid waste feedstock, comprising the steps of: a) separating the sewage sludge feedstock into a relatively high density pumpable sludge stream and a lower density stream; and b) co-composting said pumpable sludge stream and said solid waste feedstock.