Case Study SA-JW4

SOLAR HEATING AND COMPOSTING OF SLUDGE WITHOUT THE USE OF BULKING AGENTS

The introduction of the 2006 South Africa Sludge Guidelines heralded a new era in the management, treatment and disposal of sludge. Coupled with considerable increase in electricity costs from 2009 onward, thought had to be afforded to produce high quality sludge at reduced production cost. This case study describes the optimisation of mechanical assisted solar sludge drying and composting without the need for bulking agents and curing, with the sludge sourced from mesophilic digesters.

SOLAR HEATING AND COMPOSTING OF SLUDGE WITHOUT THE USE OF BULKING AGENTS

Ref / Case Study SA-JW4 / Responseinformation,descriptionandremarks
1 / Location: / JW are situated in Central and Southern Gauteng Province of South Africa. The areas are densely populated. Two of the works are situated in the northern catchment and three in the southern catchment.
2 / Sector: / Sewage sludge
3 / WorksOwnerorOperator: / The plants are owned by the City of Johannesburg (Water Services Authority) and operated and maintained by Johannesburg Water (Water Services Provider).
4 / Size: / Some 3.5 million people are served in the northern catchment and 2.5 million people in the southern catchment. Of the 260 dry tons per day of sludge produced, 200 dry tons per day are treated by the solar drying method
5 / EnergyProvider: / Power is in form of electricity, provider by the national electricity agency, ESKOM. A typical plant sized at 450 Ml/day requires 8.0 MW energy, of which sludge treatment forms the bulk of the operational budget.
6 / Process: / Operation consists of front-end loaders, sludge turners and screen.
7 / Component: / All waste activated and fermented raw sludge
8 / Motivation for the case study: / Most sludge treatment and disposal options available require enormous amounts of electrical energy to function i.e. incineration, pasteurisation etc. This process under discussion requires only electrical energy for screening the composted product i.e. 7.5 kW for 100 dry tons per day of sludge treated.
9 / Process/Plantchanges: / All plant used for composting with a bulking agent was used in the new operation. The existing inclined face compost turners were converted to sludge turners at a third of the cost of new machinery. Only 12 year old tub grinders were scrapped.
10 / Civil/PhysicalChanges: / No changes were made to the existing civil works or processes were required. At two of the works, existing concrete beds were extended.
11 / OperationalChanges: / The only process change was the non-requirement of a bulking agent and that dewatered sludge was now solar dried prior to composting.
12 / RisksandDependencies: / The composting process has been in operation in Johannesburg since 1986. The dewatered sludge was dried to the dry solids concentration as the sludge / bulking agent mixture prior to composting. Full scale trials were undertaken prior the operation being implemented at the other works.
13 / Implementation: / The project was procured via a design/build process with a respectable contractor. Acceptance testing, followed by an O&M period was included as part of the contract.
14 / EnergyEfficiencygains: / With the publication of the 2006 SA Sludge Guidelines, JW were then not fully compliant and a new operation had to be developed or the existing high cost composting operation extended. Consideration was given to other sludge treatment processes but most required high electrical energy input (heat treatment etc).
15 / Cost/Benefitanalysis: / The operational cost of sludge treatment and disposal has been reduced from R 570 per dry ton to R 270 per dry ton. The payback period of drying bed extensions etc is < 2 years.
16 / Project review: / The project has only been in operation since 2007 and as yet , only minor changes have been made to the operation.
17 / Confidencegrade: / High. Data has been well documented and verified over 2 operational years. The data shows that, to date, only a limited number of composted batches failed the guideline requirements. Such batches have either been reformed or mixed with new dried material. The operation has consistently produced a class A1a final product which is sort after by maize farmers. At present, 19 farmers use the product with a waiting list of a further 20 farmers.

The City of Johannesburg produces 260 dry tons of waste activated and fermented raw sludge per day and faces many challenges with its treatment and disposal in order to comply with the “Guidelines for the Utilisation and Disposal of Wastewater Sludge” (Department of Water Affairs, 2006). The revision of a sludge management programme by Johannesburg Water began soon after the publicationof the guidelines. Company objectives required an operation to be developed that would be fully compliant with the guidelines in a safe, cost effective and sustainable manner. The guidelines characterise the quality of the final sludge product prior to disposal by three criteria:Johannesburg Water chose the classification level A1a in order to maximise the available disposal options and ultimate registration of the final product as a sludge fertilizer.

Description of Process

Johannesburg, with its low humidity, high summer temperatures, no winter rainfall and a high evaporation rate, has an ideal climate for the solar drying and composting of wastewater sludge.The short summer thunderstorms are immediately followed by sunshine which rapidly evaporates any residual moisture.

Waste activated and fermented raw sludge are both stabilised through mesophilic digestion prior to dewatering on belt filter presses. Dewatered sludge at around 20% dry solids concentration is spread on concrete beds previously used for sludge composting using woodchips as a bulking agent. The dewatered sludge is turned daily using mechanical sludge turners and the dried sludge is removed from the beds once the dry solids concentration has reached about 50%. The drying period takes between 10 and 14 days in summer and between 24 and 30 days in winter.

Sludge dried to the required total solids concentration allows for the sludge to be heaped to three meters high without the heap slumping or forming large clods. It also allows air to freely pass into the heap, which is essential for the composting process to proceed. Above 60% dry solids concentration, rapid uncontrolled heating of the heap to above 60oC is experienced, which causes loss of moisture and inhibition of the composting process.

Under the required conditions, temperatures in the heaps rise rapidly to 40oC to 50oC and when the average temperature reaches above 60oC, the heaps are brokendown and rebuilt using a front end loader. Any sludge clods that appear around the edge of the compost heap are returned to the drying beds and broken up using the mechanical turner. After rebuilding, temperatures are observed to rise to the required average temperature and breaking down and rebuilding of the heaps is repeated if temperatures rise above 60oC. After the composting period is complete, samples are taken to the laboratory for analysis. Heaps that comply with Microbiological Class 1 sludge, are screened through a 15 mm mesh before curing. Satisfactory stabilisation of the final product appears to be achieved when the volatile solids concentration has been reduced to below 0.45 kg / kg DS.

The control of temperature and moisture content, at between 50% DS (start) and 35% DS (end), is essential for the success of the composting process. The final curing stage ensures that the final product does not cause either an odour or vector attraction problem.

The process has been implemented on four of the five large wastewater treatment works in Johannesburg since 2007.

Potential Interventions

  • Full compliance with the 2006 Sludge Guidelines at all times is crucial to the service delivery performance of Johannesburg Water and to the City of Johannesburg.
  • The low level of technology used ensures that the operation is sustainable and cost effective.
  • Laboratory analysis of all heaps ensures compliance with the guidelines and protects public health when used for landscaping etc.
  • This case present an opportunity to other practisioners that wish to follow a similar process.

Cost Saving

Johannesburg Water has reduced its sludge treatment and disposal costs from R 560 / dry ton for the composting of sludge using a bulking agent down to R 270 / dry tons through solar drying and composting without the use of a bulking agent.

Composting using bulking agent / R 560/ dry ton sludge treated
Remote farm disposal / R 360/ dry ton sludge treated
Solar drying/composting and land disposal / R 270/ dry ton sludge treated

The operation does not require the production of woodchips, removal of the woodchip after the composting process or the disposal of the used woodchip and requires handing of a third of the product during the composting process.

The 2006 Sludge Guidelines have become a requirement in the licensing of wastewater treatment plants, thereby becoming a lawful and enforceable standard. With electrical power costs due to treble over a period of 7 to 10 years, innovations such as described above that do not require high electrical energy demands may provide meaningful alternatives with higher capital, operational and energy requirements.

Reference

  • Shaun Deacon: Wastewater Engineer, City of Johannesburg