Experiences from operating the sludge dryer at the sewage treatment plant in Kłodzko

Kłodzko is located in the center of the Kłodzko Valley (Kotlina Kłodzka) and is the southernmost locality in Poland for which the company EKOTOP from Piła designed a solar sludge drying plant. The building permit was obtained in 2008, and construction lasted from October 2009 to June 2010. Official commissioning and handover for operation took place in June. The designed drying plant is a hybrid facility utilizing solar energy and heat from treated wastewater. The plant is located on the premises of the Wastewater Treatment Plant (WWTP) at Fabryczna Street, in the immediate vicinity of the Nysa Kłodzka River. This WWTP was destroyed by a catastrophic flood in July 1997, and its reconstruction and modernization lasted until April 1999. The current capacity of the WWTP is $25,000\ m^3/d$ for the mechanical section and $12,500\ m^3/d$ for the biological section. The plant's design Population Equivalent (PE) is 62,500.

The treatment plant operates on a low-load activated sludge system with final phosphorus precipitation and is equipped with aeration chambers featuring separated aeration zones. The receiver for the treated effluent is the Nysa Kłodzka River at km 128+375.

The Kłodzko Valley is the region in Poland with the lowest annual solar radiation, amounting to $996\ kWh/m^2$. Using meteorological data characterizing the climatic conditions of the Kłodzko Valley, two drying lines were designed, consisting of two halls with a total capacity of 2,500 tonnes of dewatered sludge per year. The dry matter (DS) content of the sludge introduced into the dryer is 24%, while the expected dry matter of the dried sludge is above 60%.

Wodociągi Kłodzkie (Kłodzko Waterworks) decided to divide the investment into two stages. In the first stage, one hall with a capacity of 1,300 tonnes of sludge was completed, along with a sludge dewatering station guaranteeing a high degree of mechanical dewatering of the sludge fed into the hall. The dewatering station was designed between the existing open fermentation chamber (OFC) and the drying hall.

This location of the mechanical dewatering station shortens the sludge intake route from the OFC and allows direct feeding of dewatered sludge into the dryer. This takes place in an automated system using spiral dosing and distribution conveyors. Sludge with a dry matter content of 24% is fed at the beginning of the completed drying hall, which is 120 m long and 10 m wide. A dry product storage area measuring 25 m in length has been set aside at the end of the hall. The total active drying surface area is $885\ m^2$.

Sludge drying takes place in a continuous system. The mechanically dewatered sludge fed into the hall is immediately mixed with dry sludge in the first 4–5 meters of the dryer, which is transported by a turner. In addition to its primary tasks—such as evenly distributing the sludge over the floor, mixing it, and moving it along the hall—the turner performs a sludge recirculation function that is extremely important for the drying process. This maximizes water vapor diffusion from the sludge, particularly at low insulation and low temperatures.

The described sludge recirculation inside the hall involves transporting a portion of dried sludge from the end of the hall, dosing it into the wet sludge, and mixing them.

This process is described in the German rulebook ATV-DVWK 140, "Drying of Sewage Sludge Part 1: Basics of the drying process and description of major methods," October 1998. As operational experience has shown, it is extremely important because it leads to:

* Buffering irregularities in dry matter concentration in the dewatered sludge,
* Obtaining granulate with low abrasiveness, a large external surface area, and low grain size dispersion,
* Achieving good parameters for the drying process course and obtaining a high-quality dried product,
* Enabling the binding of dust generated during complete drying, which is generally a threat to the entire process.

The adopted sludge drying technology consists of spreading the dewatered sludge in a layer approximately 10 cm thick on the floor, cyclically mixing it, and moving it along the dryer using the turner, which operates along the entire length and width of the hall. Maintaining a thin layer of sludge significantly improves and accelerates the drying process and also prevents the bed from putrefying. Furthermore, during periods of freezing temperatures, it enables drying to continue by preventing the sludge from freezing solid. The operating method of the sludge turner is shown in Fig. 2.

As a result of mixing the mechanically dewatered sludge with the dry product, an intensive drying process occurs over the subsequent meters of the hall's length. An additional advantage is that the sludge does not smear or become sticky.

The drying plant utilizes two alternative heat sources: the greenhouse effect, obtained from solar energy, and a floor heated by thermal energy generated via a cascade of heat pumps from treated effluent. Existing operational practice, both domestic and foreign, proves that underfloor heating is an effective method of supplying heat from the outside, particularly to sludge (mechanically dewatered) with a high water content (80%–70%). Sludge in a pasty form has a high specific heat, adheres well to the substrate, and has a high thermal conductivity coefficient. Thanks to these properties, it is possible to transfer relatively high thermal power to the sludge, mainly in the first phase of drying. At that point, so-called free, unbound water evaporates from the highly hydrated sludge, and evaporation proceeds as it would from a free water surface. The argument for using underfloor heating, especially during the first drying period, is that this is when drying proceeds most intensively, leading to the evaporation of approximately 50% of the water in the sludge.

The collector for the low-grade heat source is a heat exchanger located in the secondary clarifier. The heat recovered from the wastewater powers a cascade of heat pumps with a total thermal capacity of $210\ kW$, located in the building of the old sewage pumping station.

The hybrid drying system is independent and allows the user to utilize the designed systems at will, depending on demand.

Sludge drying in the Kłodzko dryer from July 2010 to October 2010 was conducted exclusively using sunlight. In November and December 2010, the underfloor heating in the dryer was activated. At the end of December, floor heating was discontinued, and the power supply was disconnected. Despite freezing temperatures in January and February, the drying process was and continues to be conducted continuously and successfully without the support of the heated floor.

In July and August, the sludge drying process proceeded very efficiently due to exceptionally high isolation and high temperatures. During these months, a series of technological tests, metering, and adjustments were carried out for both the sludge dewatering station and the drying installation. Therefore, the first two months of the dryer's operation were omitted from the analysis presented below.

The process was properly conducted from the beginning of September until the end of December 2010. Currently, the dried product obtained in January and early February is undergoing analyses to compile results and enable subsequent comparisons.

In the described observation period from September to December, the dry matter of the sludge taken from the OFC to the mechanical dewatering station ranged from 3.5% to 3.9% DS, averaging 3.6% DS. After mechanical dewatering, the sludge introduced to the dryer had a dry matter content of 24.2% to 25.8%, averaging 25.5% DS. The dry matter of the dried sludge transferred to the storage area ranged from 88.5% in September to 64.7% in December. The average dry matter value of the dried product obtained in these months was at the level of 74.1% [Fig. 3].

In the period from September to the end of December 2010, a total of over 322 tonnes of sludge with an average dry matter content of 25.5% was introduced into the dryer. The sludge was dried to an average of 74.1%, and its quantity was 112 tonnes, resulting in a 2.8-fold reduction in mass [Fig. 4].

Measurements of electrical energy consumption for drying 1 tonne of dewatered sludge showed that $17.6\ kWh/t$ was consumed in September and $19.8\ kWh/t$ in October [Fig. 5]. Electrical energy was used exclusively to power the hall's equipment: the sludge distribution system, the turner, the fans, the roof window motors, and the lighting.

In November and December, the heat pump installation was activated. Total energy consumption in November was $108.1\ kWh/t$, and in December $328.6\ kWh/t$, with energy consumption by the heat pump installation amounting to $88.2\ kWh/t$ in November and $308.6\ kWh/t$ in December, respectively. Meanwhile, electrical energy consumption by the hall equipment amounted to $19.9\ kWh/t$ in November and $20.0\ kWh/t$ in December [Fig. 5].

It should be noted here that both November and December 2010 were exceptionally frosty months with abundant snowfall. According to IMGW (Institute of Meteorology and Water Management) data for this period, the lowest temperatures in Kłodzko reached $-10^\circ C$ in November and $-25^\circ C$ in December. However, the drying process was conducted continuously, and the exceptionally unfavorable weather conditions did not lead to a stoppage of the dryer equipment or the process.

Taking into account the cost of electrical energy consumption in PLN per tonne of dewatered sludge introduced into the dryer, it amounted to, respectively: 4.4 PLN/t in September, 4.9 PLN/t in October, 27.0 PLN/t in November, and 82.2 PLN/t in December [Fig. 6].

The values given refer to the total energy consumed by the complete installation, with the proviso that in September and October, they concern only the energy consumed by the dryer equipment, while in November and December, they appropriately account for the energy consumed by the heat pumps. The analysis also shows that the cost of electrical energy consumption by the hall equipment over the four months studied was similar, oscillating around 5 PLN/t.

The use of underfloor heating in the winter months, even under exceptionally unfavorable weather conditions, enables the investor to maintain uninterrupted process conduct. The adopted drying technology allows continuous acquisition of dried product and, above all, prevents the sludge in the hall from putrefying due to freezing of the top layer of the stored bed. Of course, the process efficiency can be regulated at will, shifting the burden to the summer months without the need to engage the drying support system.

A preliminary assessment of the dried product sampled in January and February, performed in the plant laboratory, confirms that it contains approximately 60% dry matter. To ensure the assessment's reliability, samples are tested in an accredited laboratory.

Currently, drying is taking place without the heated floor support. Despite low external temperatures, the turner is working in the hall without disruptions, mixing, turning, and recirculating the sludge in a set automatic cycle.

Summary:

In the hybrid sludge-drying plant designed for the WWTP in Kłodzko, the first sludge recirculation system in Poland was implemented using a single machine—the turner—without the need for additional equipment or energy. The drying technology itself, when sludge is spread on the floor in as thin layers as possible, allows effective sludge drying even in winter months. The engagement of an additional heat source (in this case, a heated floor) demonstrated that conducting the drying process in this technology can be achieved in a continuous system, without disruptions, even in months with extremely low temperatures (November and December).

Additional support for solar drying, as the name suggests, is a supplement to the primary energy source and does not require its year-round use. In the case of a solar dryer, whose performance and efficiency depend primarily on weather conditions, it is the proverbial "lifeline" during exceptionally frosty winters or rainy, sunless periods.

The use of drying support in the case of the dryer in Kłodzko, in the discussed instance, for 2 months of the year, constitutes an undoubted increase in electrical energy consumption costs. On the scale of 1,300 tonnes of sludge dried during the year, the cost of drying 1 tonne of introduced sludge will increase by approximately 5 PLN. However, this is a fee for comfort and the certainty that sludge can indisputably be dried in winter as well, and that the process will not be stopped due to sludge freezing inside. It is also a guarantee of achieving the "ecological effect" assumed in the project, which translates into the closure (settlement) of the investment financing.

Surely, no expert in solar sludge drying technology, and certainly not my humble self, would deny that drying sludge exclusively by the sun is the cheapest method. For one simple reason—the greenhouse effect is obtained in a properly designed hall "for free." However, one must remember that the hall's size or number determines the possibility of drying the appropriate amount of sludge, and here the concept of "for free" takes on a practical dimension, which translates into the size of the investment and, consequently, the costs of its implementation. This is particularly important in later operations and affects operational costs. It should be remembered that the cost of sludge drying is not only the cost of electrical energy consumption but also other components, among which depreciation of the investment accounts for the largest share. Sometimes, an investment that appears more expensive is in practice cheaper to operate, and the very fact that a weather-dependent solar dryer can become independent of weather (through the introduction of additional heating sources) is understandable exclusively to its operator, over whom hangs—like the "Sword of Damocles"—the requirement to obtain the so-called ecological effect written into the financing agreement: that is, the drying of the entire batch of generated sludge regardless of weather conditions.

Abstract

The article presents experiences from operating a hybrid sludge-drying plant at the Wastewater Treatment Plant in Kłodzko. From June to October, the dryer utilized solar energy exclusively. In winter, the drying process was additionally supported by underfloor heating. The article presents the costs of electrical energy consumption used for sludge drying.

The solar drying plant in Kłodzko has operated continuously since its start.

Keywords: sludge drying plant Kłodzko, hybrid sludge dryer, EKOTOP, sewage sludge drying, WWTP heat pumps, sludge turner, drying costs, waste management engineering