Stratification and equalization cycles in shallow maturation ponds with different operational configurations and at different periods of the year

Abstract

This study aimed at evaluating the occurrence of stratification/equalization cycles in two full-scale shallow maturation ponds in Brazil, with different operational configurations and different periods of the year, through monitoring of temperature and other constituents in the vertical profile of the ponds. The study comprised two operational phases: one phase in which both ponds had no baffles and operated in parallel (one pond had sludge accumulated on the bottom while the other did not); and another phase in which the ponds operated in series, the first pond without baffles and with accumulated sludge on the bottom while the second pond had two longitudinal baffles, a shallower depth and no accumulated sludge. Overall, there were systematic daily periodic events of thermal stratification followed by temperature equalization and vertical mixing in both ponds and operating phases. Vertical temperature gradients were predominantly in the 0–7 °C m−1 range. Statistical tests showed a significant positive correlation between the thermal gradient in the pond and air temperature, but not between the thermal gradient and wind speed. Data suggested that ponds remained 56% of the time under thermal stratification and 44% in vertical mixing. The data also highlighted the importance of sludge in the thermal balance of ponds.

Upgrading and evaluation of a simple pond system for small communities with simple interventions to reduce land requirements and increase performance

“…Vertical temperature gradients were predominantly in the range of 0-7°C/m. The ponds remained 56% of the time under thermal stratification and 44% in vertical mixing (Passos et al 2016b). It is believed that vertical mixing enhances coliform removal, since it transports bacteria from the lower levels to close to the surface, where the inactivation mechanisms related to solar radiation are at their maximum.…”

Relationships between abiotic and biotic variables in a maturation pond and their influence on E. coli removal

“…Among the abiotic factors, pH and DO are directly related to photosynthesis by algae (Liu et al 2018) and water temperature differences leads to thermal stratifications events (even in shallow ponds), therefore decreasing E. coli abundance mainly in the surface layers (Liu et al 2018). Greater bacterial population densities generally occur at the bottom of the ponds because of lower solar radiation intensity reaching the floor/base of the pond (Passos et al 2019). Solar radiation is considered the most efficient form of coliform inactivation/disinfection/removal (Dias et al 2017b), with ultraviolet radiation A (UVA) and ultraviolet radiation B (UVB) contributing through three mechanisms: (1) direct damage to DNA; (2) indirect endogenous damage, and (3) indirect exogenous damage (Davies-Colley et al 2000;Kadir & Nelson 2014;Liu et al 2018).…”

Balanço de oxigênio dissolvido em lagoas de polimento tratando efluente de reator UASB

“…Nos dias subsequentes, condições aeróbias estabeleceram-se, prevalecendo até o fim dos 10 dias. Os valores de OD registrados permaneceram próximos ou acima da saturação durante as horas de radiação, com concentrações médias (picos) que chegaram a 25 mgO 2 L -1 no período da tarde.O comportamento cíclico do oxigênio dissolvido e as concentrações transientes de supersaturação na massa líquida, com picos de OD que se aproximam de 40 mgO 2 .L -1 , são comumente relatados em sistemas de lagoas(EL OUARGHI et al, 2000;MENDOZA et al, 2013;ARBIB et al, 2017;PASSOS et al, 2019).Essa condição é resultado da intensa produção de oxigênio fotossintético pelas microalgas, principalmente nas horas centrais do dia(ARBIB et al, 2017), que resultam em taxas de produção superiores aos processos de consumo e de transferência de OD para a atmosfera(EL OUARGHI et al, 2000).O pH (Figura 3), também governado pela atividade das microalgas (ABDEL-RAOUF; AL-HOMAIDAN; IBRAHEEM, 2012), aumentou no decorrer do tratamento, atingindo o máximo de 9,56 (às 16 h, ciclo 1), ao final dos 10 dias.Valores ainda maiores de pH (entre 10 e 11) podem ser atingidos em LPBS, a depender do tempo de ciclo, da configuração da lagoa e da sazonalidade, porém são dificilmente encontrados em lagoas convencionais, que geralmente são neutros ou levemente alcalinos(KAYOMBO et al, 2000;DIAS et al, 2014;PASSOS et al, 2019). Valores elevados desse parâmetro, apesar de poder ser prejudicial a certos tipos de microalga, contribuindo para reduzir a produtividade na lagoa(SÁNCHEZ et al, 2008), tornam-se benéficos à remoção de constituintes do efluente, incluindo os patógenos(BOLTON et al, 2010) e os nutrientes, como o nitrogênio (pH a partir de 8,5 e 9), por dessorção da amônia, e o fósforo (pH a partir de 9,5 a 9,7), por precipitação de fosfatos (VAN HAANDEL; SANTOS, 2021b).…”