Iwa international Specialist Conference

81.

Digestion of sludge and organic waste in the sustainability concept for Malmö, Sweden

Jes la Cour Jansen**, Christopher Gruvberger*, Niklas Hanner*, Henrik Aspegren*, Åsa Svärd**
*Malmö Water & Sewage Works, S-205 80 Malmö, Sweden

** Lund Institute of Technology, Box 118, S-221 00 Lund, Sweden

ABSTRACT

Anaerobic digestion of sludge has been part of the treatment plant in Malmö for many years and several projects on optimisation of the digestion process has been undertaken in full scale as well as in pilot scale.

In order to facilitate a more sustainable solution in the future for the management of solid waste organic waste is sorted out from households for anaerobic treatment in the newly built city district Western Harbour, in the city of Malmö, Sweden. The system for treatment of solid organic waste is integrated in a centralised solution located at the existing wastewater treatment plant.
Here normal raw sludge together with organic waste from industry, restaurants, big kitchens, food stores, supermarkets, green markets etc. can be utilised for biogas production and production of fertilisers, together with the household waste. Furthermore fat and oil from fat and oil traps that today is deposited at landfills are intended to be included in the process. Collection and pre-treatment of different types of waste are in progress together with examination of biogas potential for different types of organic waste. The main goals are, to increase the production of biogas, to produce fertilisers acceptable for the farmers and to reduce the amount of wet waste to be incinerated. Waste collection as well as anaerobic digestion in laboratory and pilot scale has been performed during the last year.
Anaerobic digestion at Sjölunda wastewater treatment plant

The anaerobic system at Sjölunda wastewater treatment plant consists of 6 digesters operated mesophilic (about 35 °C) with a total volume of 16300 m³. In order to reach a good stabilisation of the sludge the digesters are operated in three pairs each with two digesters in series. Different strategies can be chosen regarding how the organic waste should be digested. At the moment the organic waste are co-digested with sewage sludge, but separate digestion can be chosen as well. The organic waste is pre-treated before digestion in the two new digesters (2*2500 m³). Some characteristics of the anaerobic system are presented in table 1.

Table 1 Characteristics of the anaerobic system at Sjölunda Wastewater treatment plant

Item	Unit	Value
Total digester volume	m^3	16300
Organic loading rate	kg VS/m^3*d	2.7
Hydraulic Retention Time	d	20

Mesophilic and thermofilic digestion in pilot scale.

Pilot scale digestion have been a vital part of determining how different operation conditions affect the biogas potential and stability of the process at the Sjölunda Wastewater Plant. The effect of changing the process from mesophilic to thermophilic has been examined. Furthermore pilot scale digestion has been made in order to evaluate the effect of pre-hydrolysis of the sludge prior to the anaerobic digestion process. At the moment studies of process stability and the biogas potential of household waste are performed. Examination of thermophilic digestion has been finished and the examination of the mesophilic digestion is running.

Pilot scale testing of sludge and household waste.

5 pilot-scale anaerobic reactors can be operated in parallel for digestion of sludge, solid waste or mixtures. Each reactor is equipped with heating, stirring and gas collection. Feeding is done manually once a day. Each pilot plant system includes a cylindrical digester with a volume of 35 litres connected to a gas collection tank. The digestion reactor is placed in a water-bath heated by an electrical heat producer which mean that the reactors can be operated mesophilic or thermophilic.

Figure 1 shows one of the five systems together with the operational parameters for thermophilic digestion of household waste.



Operational parameters:

Solid retention time: 15 days

Temperature: 55 ºC

Dry matter content in feed: 5 %

Organic load: 2,8 kg VS/m³·day

Feeding/Residue removal: Once every day

Figure 1. The system for pilot-scale anaerobic digestion with chosen operational parameters for thermophilic digestion.
Anaerobic digestion of household waste.

In the new city district two new collection systems are tested. Food waste disposers are used in a small area and a mobile vacuum system is used in general (Gruvberger et al, 2002). The two systems represent different ways of sorting out organic waste. Biogas potential are examined in laboratory scale and pilot scale testing is performed for evaluation of process stability. Thermophilic as well as mesophilic conditions are included in the examinations.

Figure 2 shows the gas production from thermophilic digestion of household waste pre-treated in a screw press. The gas potential found from the accumulated production of methane versus the inlet amount of organic material (volatile solids) during ten days after stable operation is reached. Description of the start-up procedures and results can be found in (Svärd, 2002).
In order to evaluate the potentials for co-digestion of sewage sludge and kitchen waste, a pilot scale test was initiated where one digester was fed with sewage sludge and the other with sewage sludge and organic waste from waste disposers. The organic waste constituted 20 % of the VS-load. Both digesters were operated mesophilic with an HRT of 12.5 days and a VS-load of 2 kg VS/m³·day. Preliminary results have shown that the co-digestion process remains s
table and shows a higher gas production than the sewage sludge process as seen in figure 3.

Figure 2. Methane production vs. accumulated amount of VS for thermophilic digestion of household waste.

Figure 3. Methane production vs. accumulated amount of VS for mesophilic digestion of sludge and a mixture of sludge and household waste.

The full paper will present more results from separate and combined digestion of sludge and household waste together with gas potential measurements of different types of organic waste. Mesophilic and thermophilic digestion will be compared and separate digestion versus co-digestion of sludge and organic urban waste will be will be discussed. The potential for production of fertilisers from the organic waste will be evaluated.