Overall, the framework conditions in South Africa for the generation of electricity from sewage gas are rather difficult. The remuneration for feed-in from renewable energies is fixed at auctions, this procedure prefers scalable technologies such as photovoltaics. Biogas, on the other hand, is limited by the availability of raw materials. This restriction applies in particular to the conversion of sewage and landfill gases into electricity. The unique selling point of biogas, simple storage and power generation at peak times, is also not affected by this process of auctioning.

Thus, the only economically decisive factor on the South African energy market is the savings in own electricity consumption. The current electricity prices of around 1.20 rand/kWh (around 0.08 €/kWh) are low by German standards. However, the electricity price has multiplied since 2007: at that time, it was 0.013 €/kWh. In addition, annual price increases of 10% have been announced.

In this field of tension AKUT was commissioned by the Deutschen Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH to prepare two studies on the use of sewage gas for the two sewage treatment plants Kingstonvalle and Zeekoegat.

Economic plus ecological advantages

Against the current background of water scarcity in Cape Town, the relevance and explosive nature of the topic becomes very clear. The lack of rainfall is primarily attributed to climate change. The studies therefore identify emission reductions, in this case reductions in methane emissions, as a significant advantage in addition to economic advantages. At present, the gas produced by sewage sludge digestion is released into the atmosphere without further measures. This is now to be changed so that the gas is collected, processed and used for power generation.

The studies present detailed comparisons of various technical components, operating models and organisational forms. In both sewage treatment plants, the economic benefit can be maximised by optimising the utilisation of the existing digestion towers. Capacity utilisation is to be increased by adding so-called co-substrates, either digested sludge from neighbouring sewage treatment plants or waste from the agricultural industry. The latter could be identified by local AKUT employees so that specific modelling could be carried out, taking into account both the biogas potential and transport costs.

Economically reasonable are also the variants with an exclusive treatment of the sewage sludge of the sewage treatment plant. However, the then lower investments will lead to a slight reduction in profitability.

In addition to the technical components and the various operational management, operator models from in-house operation to BOT models were also examined. Two preference variants were determined from six different operator models.

The decision now lies in the field of politics. Should the measures be implemented? Which of the preferred variants is preferred? GIZ and AKUT are ready to continue to support the City of Tshwane and Mbombela municipalities – both in decision-making and in technical implementation.

The international IWA conference on sustainable solutions for small water and wastewater treatment systems (s2small2017) was held October 22-26 in Nantes, France. Florent Chazarenc (IMT Atlantique) chairman of the conference emphasized that small systems play a significant role in solving problems of humanity: „Small is beautiful, efficient and affordable – small is the future“ he said. More than 200 participants responded to the call and discussed their proposals for solutions in the areas of resource-oriented sewage systems, wastewater treatment and recycling.

As a German contribution to the improvement of small sewage treatment plants Heribert Rustige, AKUT partner from Berlin, presented the new DWA worksheet A 262 on the topic of constructed wetlands. This guideline is expected to be published in November 2017 and replaces the previous A 262 from 2006, whereas many new solutions and system variants have been added to the new worksheet. This includes the so-called French system, which consists of a combination with raw wastewater pre-treatment in a planted gravel filter and post-treatment in a planted sand filter (e.g. Phragmifiltre). Another newly described method using active aeration is particularly innovative. It was developed by Scott Wallace in the USA and extensively tested on site for several years in Germany by the UFZ research institute in Leipzig.

International Interest in the German Constructed Wetland Guideline

In the past, the A 262 had already attracted interest from other countries. It was last translated into Russian. Many users are now waiting for the release of the current version.  An English version will also be available soon from the DWA in Hennef.

The advantage of such a detailed set of rules is the increased safety for planners and operators. The worksheet specifies minimum requirements for the dimensioning of soil filters and refers to values derived from practical experience.  The user must check in each case whether the boundary conditions are correct in his application. In doing so, the regulations focus on the treatment of domestic and municipal wastewater, including combined sewerage systems. In addition, the treatment of grey water in soil filters is also taken into account.

The tables, which describe, for example, the different wastewater compositions or specific design values, are helpful. Various useful combinations of processes are shown. However, Rustige emphasized in his speech that the guideline is not a simple construction manual, as it requires specialist knowledge. No design formulas or modeling approaches are shown because they have not yet proven themselves in practice.

What is more interesting is the presentation of minimum requirements with which certain effluent values or performance degrees can be achieved. In the theoretical modeling of treatment efficiency, it is often not taken into account that hydraulics or oxygen transfer rates can be decisive limiting factors, which in the worst case can lead to a total clogging of the soil filter. It is better to orientate oneself on the statistically relevant results derived from practice, as they are used in the A 262.

The municipality of Theres, Unterfranken, Bavaria, was awarded the Bavarian Innovation Prize 2016.The new groundbreaking concept for the replacement of an aged waste stabilization pond which was developed by AKUT will obtain a natural way of treatment combined with an inexpensive way of operation.